As much as I like the base-game, it's severely limited in the late-game sections of it and it quickly becomes untenable for players to maintain huge (500+) populations in it without this DLC. Now, I'm not saying the devs knew this from the get-go and deliberately made this DLC to sort that issue out, but... okay, actually, that's not far from the truth, but bear with me on this one, k?
So the thing was that the devs *did* sorta know that the late-game aspects of the base-game kinda sucked for huge populations, and they *did* fix a few of these issues with just regular ol' content patches, but for true veterans of EZ:AWA, Prosperity really is a must-have to add some proper late-game content.
Just the visual upgrades to your settlement are a pure freakin' delight when you get up to the top-tier stuff, but their upgraded functionality is so damn nice to boot, all o' which gives you this awesome sense of "Hell yeah, I'm the king of the wasteland, baby!"
But it also gives you a nice set of new challenges, like the new resources you gotta manage (concrete, glass, etc.) and so on.
So all in all, I'd say this is one of those DLCs that reach the way too rare heights of quality, like "Gods & Kings" and "Brave New World" (Civilization 5) and "Green Planet" (Surviving Mars), the kind that adds a whole lot more to the game for a reasonable enough price.