Tony Vargas
Legend
Nod. Gygax was looking for a model of magic that'd work in his new fantasy wargame, that would allow magic-users to participate in combat rather than the more traditional visions of magic (that, in today's D&D would be ritual casting) and he came up with the 'magic' Jack Vance used in his science-fiction classic, The Dying Earth: relatively short spoken spells that were 'memorized' and 'forgotten' when cast. That was a choice of concept to model based on the qualities the mechanics that modeled it would have being theoretically good for what he wanted. Like the Dying Earth, D&D magic-users started memorizing very few spells at a time, but as levels got higher, they went off the rails.The discreteness made perfect sense in 1E-3E where you were memorizing/preparing spells individually. It's a little fuzzier what's going on in 5E.
'Memorization' drew a lot of flak over the years so the same mechanics were re-imagined as less-preposterous-seeming 'preparation,' from that point on, it seems, mechanics have driven fiction.
But, that does leave it open to DMs choosing various fictions based on those mechanics...
OK, I'll think of it that way: In that analogy, a 5e prepared (known) spell could be the circuit, and the slots would be capacitors, that for whatever reason, only come in certain arrays...It's not quite as "clean" as it was in 3, but I wasn't able to come up with a better alternative.
Think of it like wiring up magical circuit. Casting the spell, completing the magical circuit, doesn't mean you don't still have the circuit available for use later as long as you have enough juice in the reservoir to power the result you need.
...yeah, that kinda works.
Traditional Vancian is conceptually simpler, and the bookkeeping also seems as simple or simpler: you have one list of prepared spells, as you cast them, you cross them off, vs a list of prepared spells, and a separate pool of slots that you check off. Neither is particularly intuitive to the new player, I wouldn't think (it certainly wasn't back in the day).I agree that old-school Vancian is conceptually simpler, but there is more bookkeeping, tracking what's going on with each individual slot rather than just having a list of things you can do. 5E places a high premium on ease of use at the table, so it makes sense they went the neo-Vancian route.
Vanican, however is much less flexible and harder to use optimally in play. More challenging, in spite of being less complicated. Neo-Vancian is easier/more-forgiving and delivers a more powerful magic-user, as it makes it easier to bring the optimal spell to bear in each situation - it's less challenging, even though somewhat more complicated.