Alright, I'll give it a try in a bit when I can get back in game.
As to why, it purported gives a cleaner, sharper image. Here's an interesting thread about it
at Neogaf. I just got a new video card that is capable of it, so I wanted to give it a test drive and see for myself. So far, the actual game does in fact look very nice with the effects, but the UI definitely seems to struggle with the very high resolution.
Thanks for the response. I've been using RealUI for most of Cataclysm and all of MoP, and I really love it over managing a bunch of addons myself. Thanks for the work you guys do.