Created by apprentices, or by mages who can't cast spontaneous spells, or by a mage who needed higher Penetration than he would be able to get with a sponted spell.
Some might have been created as part of Original Research.
Lots of possible reasons for inventing low-level formulaic spells.
The biggest contributor would be when someone has just enough extra lab total to say “I think I could invent this 1st magnitude spell after I’ve spent the bulk of the season inventing the spell I really needed to invent.” Some others likely come about from folks with the Faerie-Raised Magic virtue and some might come about using the optional rules in Apprentices, Canticles or whatever they’re called.
Yes, you're inventing a level 30 spell and your lab total is 50. You could accumulate 20 levels the first season and the second season you finish you level 30 and have enough lab total left over to make a level 5.
Consider also that the first mages of the order, even if the founders are semi-mythical, may not have been that great of a magi. They had no summae to learn from and probably depended a lot more on vis study and adventure. They also had precious few lab texts to use. There's a good chance that most level 2-5 spells are usually sponted, yes, but sometimes you don't want to wait until your casting total for InVi is 25 to figure out the type and quantity of vis in an item since you'll probably need to use that vis to study to get their art scores beyond 5. Yes, chances are that most standard level 2-5 spells were the first spells that got researched and distributed.
People gave you great ideas above, and let me give you another example.
I have a Magus with 11 in Ignem, and had 1 in Perdo. Our covenant has no Summa or Tractatii on Perdo, but I want to improve it, for various reasons. I researched a PeIg5 spell that allows me to put out a fire by touch. Not a great spell, but it increased my Perdo from 1 to 2. Since my Lab Total is a bit over 20, I also researched another PeIg spell that drains one fatigue by touch, due to cold. So, I got 2 spells, and Mastery 1 in those spells, all in one season.
So 22 XP worth, in one season.
Left over Lab Total is by far the most common I have seen in every saga I have played in. If the Magi in the Covenant write up Lab Notes on them and submit them to the Covenant library then they will rapidly spread through all the Magi. How often do you have 5 extra points of Lab Total after inventing a spell? With a Lab Text you can learn a level 5 spell with that leftover.
The advantages of knowing rather than sponting low level spells don't just include penetration. They are also much more resistant to adverse casting conditions. What you can non-fatigue spont at home in you magic aura is very different from what you can non-fatigue spont in an adverse aura. This is a big one since it can allow you to reliably cast low level spells in conditions that you would have to fatigue spont or possibly not even be able to spont it. Knowing the spell is far better than having to fatigue spont it.
If you are at all studied in the Arts of a level 5 spell, you can often cast it silently and without gestures (-15 to the Casting Total) if you know it. With a 5 in each Art, a Magic Aura 5, and Sta +3 you would need to roll a 2 to do it. Slightly better Arts lets you do it outside a Magic Aura. This is something you most likely can not do with a spont (Arts + Sta + Aura of at least 40 for non-fatiguing). So learning it gives your Magi a magical effect they can just do by concentrating on it. Worst case they might have to whisper a quiet word (reduces penalty by 5) or flick a few fingers (reduces penalty by 3).
While that CrIg specialist might be able to light things on fire just by touching it (Base 4 "Ignite something flammable" +1 for Touch), learning a spell would allow nearly any Magi to do the same thing. If you learn it with what would be wasted excess Lab Total, this is a nice little "innate ability" to round out your Magus.
You also can get Spell Mastery in a learned low level spell. While not generally worth it, if you have Flawless Magic it can get pretty nutty real fast. Spending 5xp for Spell Mastery 1 though can be really worth it if you plan on casting the spell in a non-Magic Aura. Removing the chance to botch is powerful. Unless you have really high Arts you would have to fatiguing spont the effect and non-familiar Aura make botches much more likely.
You do not even have to target Level 5 spells. There are published level 3 and level 4 spells. You would need a total of 30 or 35 in (Arts + Sta + Aura) to non-fatiguing spont these silently and without gestures. If there is some little effect which you feel would fit into your Magus' concept then pick it up.
Final thing. General spells which take advantage of penetration can be really nasty when learned at level 5 and mastered. Multicasting DEO level 5 or its variants can rapidly whittle down even fairly strong opponents.
I agree very much with @InfinityzeN . I think he covers most of it.
It strikes me though, that there are both IC and metagaming reasons flying about.
Left-over lab total is pretty obvious from the game mechanics, but I do not really believe that real wizards reason that way. Wouldn't they rather take Saturdays off and listen to music or go hunting? Or if they really are monomanic lab rats, they might start a new high-level project.
In character I believe much more in early-stage researchers who want the convenience spells years before they learn the arts to spont without stress.
Some of them may be mere demonstrations of features (guideline in game terms) previously not commonly known, i.e. they serve to prove that you can spont it rather than encouraging you to learn it instead of sponting it. Others have already mentioned spells originating in traditions without spontaneous magic, or in the early Order with weaker magi.
Some may be by-products of original research, either because the researcher is risk-adverse or because the first magnitude spell was the best idea they could come up with that season.
Left-over lab total makes sense to me - if you're already in the lab, studying similar things, why not spend a bit of time learning a quality of life spell? You might never get around to it otherwise. It's basically mucking about with code for magical programmers
Sure, but that same logic also applies to the whole season (do I really need this spell that's going to take months to learn?). Note that I'm not suggesting that wizards won't sometimes just take extra capacity as an excuse to wrap up early, but the "while I'm here" logic makes sense to me
How long do you expect it to take me to cast this spell? We're talking spells, not rituals here.
And I know more than a few people who joke about how they tend to spend hours generating (writing) tools to automate/resolve repeated minor issues that take minutes to deal with.
I could see going different directions. Yes, there is the improvement on quality of life that pushes people to work on these things. At the same time, we can look at real-world trends. People tend to behave a bit opportunistically when things are convenient even when it costs them some time and money, such as buying extra things when shopping. You're already in the lab, why not take a few minutes here and there to get this other thing done. And then there are people who don't behave this way. People also tend to avoid major efforts that aren't their big things. For example, lots of people put up with a lot going to work each day, while many of those same people won't put in the same sort of hours at home fixing up their place to improve their quality of life. And then there are people who behave opposite to this, who try to get lots of break at work but will take care of stuff at home. So I can see it showing up realistically both ways.
I think it's wrong to consider this as player characters trying to optimize experience points.
In game there are hints that characters might be aware of what the level of a spell is (or at least the magnitude), but XPs needed to research a spell are a character mechanic, and thus a gaming tool, not something inherent to the characters mind frame. So considering this I think many low level spells come from NPC magi working in a sub-optimal way.
Also I like to think that magic is evolving and changing, and part of these changes are advancements and discoveries that from time to time reduce spell base levels. Many current low level spells might have been higher level when they were first created.