Wizards conduct their adventures with caution and forethought. When prepared, they can use their spells to devastating effect. When caught be surprise, they are vulnerable.
OK. Now the spell memorization/preparation thing from D&D makes more sense to me. It is the magic-user’s Achilles heel. An exploitable weakness.
I mean, I understood it as a game mechanic. A spell-point system with some extra rules that meant giving the character the ability to cast one very powerful spell didn’t give them the option of casting a lot of low-level spells in exchange. But now it makes more sense with this tidbit.
My group subbed the word “preparation” for “memorization” in the 1980s.