There exists a specific game mechanic for spell casting in the Dungeons and Dragons system (at least in version v3, v3.5 and also in Pathfinder v1, probably in others also) which seems to be driven by ludic / balance reasons mostly.

I'm talking about the mechanic which..

• allows Wizards classes to learn a great number of spells, but..
• forces them to remember/prepare/imprint¹⁾ a chosen, limited number of spells for daily use.

From a game mechanic standpoint I can see where this is coming from: it allows Wizard characters great flexibility in their spell repertoire, while still forcing their players to make tactical choices which will heavily impact how the Wizard performs in specific situations, e.g. in battle or when faced with other challenges. The mechanic has been around for a long time, and, I guess, has worked fairly well.

Is there an in-universe explanation for this mechanic, either from any of the DnD worlds (e.g. Forgotten Realms) or from the Pathfinder side? It seems rather unnatural and arbitrary after all, that a Wizard may know 20 spells of, say, level 2, but..

• A) can only cast a small number of them daily (even if the same spell!) and..
• B) the small number of spells can be completely changed overnight.

¹⁾ I'm not sure about the correct terminology here, I don't have any rules at hand...