I was engaging in some intellectual exercise the other day and creating the ultimate labrat character. I end up with a Lab Total that was a full 14 points above my casting total. More may well be possible, but this character was still playable, which is also important.
But my question is this: Is there a point in having a lab total that high? She can learn spells she can't cast with that kind of total. I can see it would be useful for enchanting items, but is there any benefit otherwise, save perhaps crafting spells for the covenant as a whole or for hermetic acclaim with House Bonisagus?
You can learn a lot of spells you can cast with that casting total. As long as you have the appropiate tech+form combos of s`pells in the covenant library, yeah, sure. This character is bound to advance quite rapidly at the start of the campaign. Later, it might fall back on track with his/her peers, but it is a useful character.
I think the point of having a high lab total is for the character to create his own spells, as opposed to learning them from lab texts. (If you're playing a lab rat, it's usually because you want to do New Stuff, right?) In that case, having a lab total dramatically higher than the casting total is fine and dandy as long as the casting total still exceeds half the lab total (i.e. the level of the most complicated spell the character could create in a season). In a character's specialties, this is probably the case.
Nonetheless, a lab rat could learn spells that he would find it quite difficult to cast if working from a text. I take it that this won't happen often, though. Mercurian wizards might be an exception due to Wizard's Communion.
(For what it's worth, my current PC is a Bonisagus lab rat. Due to this sort of thing, he has - once - learned a spell that he had a great deal of difficulty casting, a version of The Aegis of the Hearth. We had a couple of tense years before he went on a crash course to study philosophiae.)
Lab total is used in a number of ways relative to (spell or device effect) level:
to invent a new spell or effect from scratch, you need to exceed the (spell or effect) level, and accumulate those points until you accumulate as many as the total level. (Thus, if you can double the target level with your Lab Total, you can complete the work in 1 Season - just like a Lesser Enchanted Device)
high Lab Total both makes the design possible, but also makes it faster!
If making Charged Devices, you get (nunber of charges) = (Lab Total - design level)/5, so high Lab Total means more charges
if working from a Lab Text, it's a bit like the text removes the subtraction of design level.... Thus you can learn a spell in just one season if you can match the level, or replicate a device effect, or make Charges = (Lab Total)/5
you can work on multiple, lower level activites in a single season, if you have a high total. So long as the activities are the same type (invent spell, invest device,...) and the same TeFo, you can split the Lab Total between the activites however you desire. (Multiple spells, or device effects, or charges).
Lab Total is used in other important lab activites, like Longevity Ritual design, and bonding Familiars.
Note also the various ways to get even more Lab Total, like Shape & Material Bonus, Verditius Craft Magic etc. Magi have a lot of incentives to boost Lab Total...
Note also that in 5e, there is a powerful incentive to learn and use low-level spells, as then your spell penetration is much higher:
Pen = (casting total - spell level) + (Penetration Ability * sympathetic magic multipliers).
If you want a fiery bolt to burn a being with MR, you need to get high pen - and the best way is to throw low-spell spells (smaller damage - but hey, they get through, which is more than that BoAf you can only just cast...)
It occured to me that a really high lab total reduces the learning time rather than making you a more able caster. Different advantage, but in a game where time matters it's a substantial one.