I'm not asking about the rules or concepts of spell preparation, as they are well described and reasoned, but I'm wondering why it's even necessary, from a logical (well, however logical you can be in a fantasy RPG) POV.
Why do spells need to be prepared? Why can a fighter or a rogue use insta-abilities whenever he wishes, but a mage/cleric/palading can not?
I always found the concept of preparing spells very very confusing and very hard to explain to new players. It's just not intuitive.
What I can understand however is, that you can only cast a certain amount of spells during a day. That's okay, as I can imagine casting to be very, very exhaustive (mentally, obviously).
Why not just say: "You can cast 1 spell per day" and be done with it? Which one it is doesn't really matter in the end, because you are ALREADY limited by the amount you can cast.
Furthermore; wouldn't the Mana concept be even easier? Having some points that regenerate while resting, and every spell having a different amount of needed points?
Why limit it twice?