Spells: preparation vs. improvisation
Everybody has an opinion on the so-called Vancian casting system of classic D&D. Some (myself among them) find the idea of "fire-and-forget" spells to be a poor representation of the kind of fiction we want to represent. Some (again, including myself) find it too rigid, and dislike the incentives it provides toward spell selection. And then there are those on the other side of the fence, who like the in-fiction implications (still not a fan) or who see value in such a system for the purpose of promoting game elements of resource management and player skill (for which I do have an appreciation.) Not a whole lot need be said about the in-universe implications of spell preparation. It could literally be a case of impressing patterns of magical energy (or imprisoning a minor daemon or other entity) in the mind and then discharging it, losing the pattern or knowledge until it is prepared again) but it could just as easily be reskinned as carefully preparing proper reagents,