I split this out of the Question related to ring duration
thread since it was so different
If they are a common item you may beable to find/trade/buy a lab text that lets you cut the difficulty way down.
You can put triggers into them which otherwise can only be done with a watchingward type of spell(which needs vis)
I'm sure others can give you more reasons
No need for casting rolls and thus no possibility of botches!
Allows the magus the freedom to do something additional in the round depending on how you design the trigger and what the particular effect happens to be
They are a commodity which can be sold or traded, thus adding to personal or covenant wealth and resources
They make a magus look stylish (important for magi from Milan and Paris especially) 
Toys are just plain fun!!!
Our reasons so far, all lesser enchanted items at 1-2 pawns vis each.
They make good gifts (e.g. lesser enchanted, 70 year effects), e.g. a wooden pedestal enchanted so that a dragon appears rearing up on it to command (a present to a lord obsessed by dragons).
They work and can be activated when there are no magi present.
e.g. a pomander of detecting the scent of vis to a travelling companion. An vis source after a fashion.
A flaming sword as a gift for the mercenary companion.
They provide a useful barter tool for favours between magi in our covenant, (I'll make a useful toy for you, if you make my longevity ritual for me).
Those have been my reasons to date.
Looking forward to making my Talisman. Botch free personal effects. Link several to the same trigger phrase (defence shield active type thing).
More subtlety in casting. None of the chantey hand wavey stuff for me, not any more:).
Add to which are items that are worn which provide constant effect bonuses. Which if my understanding is right, only force warping on you if the spell is on you.
A robe enchanted to be armour would not cause warping, if it is also warded against fire, lightning etc. then I as a SG would also rule that it provides a good chunk of protection to you inside it.
Don't ignore the WOW factor either.
IMS my magus has a skeleton of a warhorse he has enchanted to be a) animated, b) cloak itself in illusions to look like a normal horse, c) fly and the last effect is only partially enchanted so far but d) teleport horse and rider.
Sure its flashy and over the top, but its so cool. Flying to your location on a skeletal horse reeks of awesomeness.
Other items he has made are a death effect wand (only works on humans) with a massive penetration (far beyond what he could cast a spell at), a constant effect armlet that protects him from one wooden and one metal attack each round (which has saved his life many times now) and a bed that provides a massive healing bonus to those that rest in it.
Importantly the bed allows for magical healing even when he (the primary healer) is a) not there or b) the injured party. It to has proved extremely useful.
Other examples are the captain of our turb who wields a magical lightning spitting sword (no penetration, so it doesn't matter if he gets mentem'ed against us), a magical plow that greatly reduces the time required to plow a field, a magical nail that, when hammered in a wall, prevents all animal and herbam products in the room from going off and other conviences.
Being able to produce a handy magical effect without a magus being present is very handy.
Magic items can also be used without effort in hostile auras, where casting spells can be challenging. Since this includes the divine auras of human settlements, that can be quite important.
We have a few investigation magic items (variety of intellego spells) with discrete activation methods to be used in crowds and in sites like churches. Our mages could not afford to cast those spells without talking and gesturing in those negative circumstances AND not fatiguing themselves.
So, appart from the magus-less party that needs some magical muscle, it is useful for mages that are not THAT powerful. In the life of a covenant that means most of the time those will find some use. When the first generation magi do not need the trinkets anymore, there is likely to be a second generation of magi that can find them usefl. Or the grogs. The only moment they wioll be wasted is in Winter ruins, and by that time they will have already been sold or in a state of waste, like the rest of the covenant.
Cheers,
Xavi
Charged items allow you to hit significantly above weight.
Let's say that you're a fairly young magus with a CrIg casting total of 30+stress, penetration of 2, and a CrIg lab total of 45. Let's also say that you've seriously offended an archmage. You could cast a Pilum of Fire yourself with a penetration of 12+stress. Just hope you don't botch. On the other hand, with no vis and a single season of work, you could also have a single use Pilum with penetration of 50. With your lab text and a second season of work, you could create an additional nine of these things.
You can make a very powerful effect (overall level higher than 50) and it's not a Ritual.
In the new rules any time a spell reaches level 55+ it automatically becomes a Ritual even if it has no other Ritual requirements. So if you want to make the most powerful Demon's Eternal Oblivion you can, it's still only level 50 unless you want it to be a Ritual.
You could make a wand with a level 75 Demon's Eternal Oblivion though, and still make it with a huge penetration compared to what is likely possible when trying to cast a 75 spell.
An item can't turn any other Ritual into an effect though (no Boundary range for example).
Unless you have been initiated into the Hermetic Empowerment Mystery. 
Obviously, you don't have a skilled Verditius in your covenant....
Perhaps I could interest you in some quality items?
In our game, it seems like most of our magic items fall into five camps:
One is covenant improvements, things that non-wizards use to do stuff or constant effects that one doesn’t want to cast over and over—items of convenience. This can include things that specialists make for non-specialists (our Braziers of Warmth, or Chamber Pots of Convenience). It also includes items purchased (or otherwise gained) from other wizards to fill gaps in our power (a good cloak of invisibility, or Cloak of Black Feathers).
The second is high penetration. My magi has weak magic, so he’s put a couple effects into his talisman with boosted penetration. Likewise when going up against something that may have once been a god we decided to spend a little prep time in the lab making a powerful limited duration device.
Thirdly there are items with powers that you don’t want to roll a stress-die for or that can take over concentration for you. For instance we have a flying broomstick which can operate without the wielders full concentration, allowing him to do other things (or take long distance journeys on it). We also have some devices to discretely work on mundanes.
Then there are the items which are gifts or aids to the companions and grogs to help them do their jobs in absence of the wizards. We gave our tame noble a little something to (discretely) improve his defense in combat, and, since he is a little young, something to dull his more exciting passions.
Finally, there are items which are poorly decorative, magic as art. In our game, some of these have been inherited from NPC covenant founders. Some of them are borderline practical (the wall in the underground lab that has a powerful illusion of being outdoors or the device that conjures a Chamber of Spring Breezes down below as well)—but don’t have any mechanical effect. These, of course are the rarest, as they take time and vis and don’t do anything outside of improve the mood of the covenant.
OK, many reasons for me to get into my lab more often.
I've constructed a talisman already and have found it lifesaving that it can teleport me to save places.
But to take a ReAq lvl. 15 effect (which is my standard damager):
My ReAq casting total: 31, penetration 2
ReAq lab total (incl. apprentice and +4 form bonus): 60
So I usually have a penetration of 18+stress die
A one-shot item could give me 90 penetration?
A 3x/day (+2) item, in one season, would give me a penetration of 26. (because my lab total needs to be twice the items level to complete it in one season) Let's give it 3 seasons of work: 56 penetration, that's better, but takes 3 seasons and 5 Vis.
How does this lab text work, so that you can make another 9 in one season. (because that really would help)
Refer to the section titled Using Lab Texts. "If the Laboratory Text is for a charged item, the magus produces an item with a number of charges equal to one fifth his Lab Total, rounded up." This is a farily powerful rule which (IMS, at least) doesn't get a lot of attention.
This lab text rule makes for loads of items (or uses) in a few seasons of work: let's see what that can mean for grogs
Single-shot items, based on lab totals less than 50.
Unholy inferno (CrIg)
This slightly burned whitewood (which translates to firewood in Dutch) wand is pointed at a group of people while speaking “infernos aut inferno.†A multitude of raging flames shoots forth, splitting in up to ten flaming spheres before exploding in an equal number of panic-stricken faces.
This single-shot item deals +30 damage to a group of targets at vision range.
Lvl. 45 (base 20, +3 vision, +2 group, no penetration) <wand = +3 to CrIg lab total>
Sacred haven (ReCo)
When the small bottle of holy water is pulled from its necklace, the wearer is instantly teleported to the church font (where the holy water was drawn from = arc. conn.).
Lvl. 40 (base 35, +1 touch) <necklace = +2 to ReCo lab total>
If you have a Verditius handy, their lab totals for crafting can far outstrip anyone else's lab totals due not only to their Craft score, but also because they usually devote Virtues to pumping their lab totals at a higher priority than most others.
A Lab Total of 60 instead of 50 is a significant jump when you're talking about most items - a permanent 40th level effect is now 2 seasons instead of 4, or a temporary 50th level effect is now 1 season of work for 7 years duration instead of unable to be made.
Before this becomes true Threadomancy, and because this question seems to get asked regularly, I'll incorporate BoXer's list and other contributions into one summation...
[color=indigo][size=150]Top 10 Reasons to Create a Magic Item instead of a Spell...[/size]
Because...
Successful Casting is GUARANTEED, regardless of any penalties.
No Fatigue is ever, ever lost. (No mage ever fell unconscious from activating an Item effect.)
Botches are not a possibility.
Proper Trigger can allow a Mage to do something else (or nothing at all!) while the effect is cast from the object. This can create double whammies in combat, discrete casting, etc.
Item can maintain Concentration for magi.
Items can be automatic, triggering without intelligent supervision, and so are perfect for traps/alarms/wards/defenses, and/or if mage is absent or unconscious.
Allows non-magi to cast spells - companions, (shield-)grogs, consortis, familiars(!), mundanes, anyone...
Constant Effects. Through sunrises and sunsets, year after year, just keeps going and going and going...
Huge Penetration bonuses are possible. Huuuuge.
While selling your "services" could be a crime, humiliating and possibly dangerous, selling an item (if done properly) is simply good bussiness.
Proper Shape/Material Bonuses, and certainly Verditius secrets, can make Items easier to research than spells for some effects/magi.
Add to that the "Flash/Fun" factor, and the fact that it's a genre tradition, and you've got your answer. Not that it's right for everyone, not that there aren't obvious drawbacks and other, negative considerations, but that's why it's right for many.
What about if a magus puts on his enchanted easy snooze nightcap?
Cucuhound summed it up nicely IMO.
Xavi
Thanks. I've been off-line for some months, and actually had been thinking about this topic separately, so it seemed too inviting. Ah, some of the other threads, now long asleep, that I would have loved to add my 2 pfennigs to... 