Exactly what it sounds like.

Just what the fuck is the point of preparation?

In 3e, it was a minor limitation on a wizard's power. A poorly-prepared spell selection meant you were less powerful that day, and wise foresight made you more powerful. That's a strange thing to reward, but maybe it's a wizardly thing. Okay, whatever.

In 5e, preparation is far less limiting; you can still prepare a bunch of spells, but now you're not preparing one slot at a time. You're basically crafting a slightly smaller sorcerer out of yourself. That's not particularly limiting, but still just as tedious. So...why?

Looking at the numbers, a fairly typical 5th-level wizard has 14 spells known, 10-11 prepared, and 9 slots total (4-3-2). So, right off the bat, I feel like having 9 slots/day is the limiting factor here--it's not like you can cast all 14 of your spells with 9 slots. But preparation limits that...from 14 down to 10 or 11. Um...okay.

It just seems like it's not doing enough to justify its existence, especially in a system that's so insistent on simplifying mechanics that fighters and wizards have the same base attack bonus. They were probably wise to take out bonus slots for high Int, and it is true that 3e item crafting made it way too easy to trivialize preparation and spell slots entirely, but while they were pruning, they should have just axed preparation too.

The mechanic makes sense for classes that know their entire list, of course. A 5th level cleric has 49 spells known, so preparation is doing its job filtering things down. So that's it then? Keep it around for clerics and druids?

Maybe. But it's worth asking "Why do divine casters know all spells?" What does that represent? Why should they have greater versatility in casting than a wizard? What, because their spells are weaker? Personally, I'd rather have equally powerful spells and suffer the indignity of having to learn spells. I mean, one of the great joys as a wizard is finding a spellbook. Why deny that to divine casters?

Let's just nuke preparation and ensure that all casters have a limited number of spells known.