If you're in this thread "I built a PC, all my stuff has RGB, it's fine but I don't get it", try setting them all to the same color - you might be surprised how nice it looks. I set everything to white, but my roommate has a nice shade of purple he uses for everything. My real golden rule is absolutely no motion. Can't stand cycling rainbows or whatever.
In terms of the "why" for this kind of thing, I guess if I'm going to be staring at a computer screen and computer all day, I want it to look interesting. The P3 case is open with a large glass plate on the front (look it up), and I've got it mounted directly on my wall. It does nothing for my computer's performance, but it makes my time at the computer more pleasant.