For Rastafarians it's religious (don't ask me the specifics, I don't know, just like I don't know why monks tonsure their hair).
For others it can be for any reason - some people do it because it works well with their hair. I have a few African-American friends who have dreads because they're actually easier to maintain than a 'fro, they don't want a clipper cut, and straightening their hair with toxic chemical to fit a mainstream ideal of beauty isn't something they're willing to do. They also tend to be better maintained than those ratty dreads you mention.
Then there's people (like my boss, and one of my "boys") who wear them as fashion. They like the way they move when they're dancing, they like how they look, it's just another hairstyle.
Then there's the nasty hippies who have the big, ugly, knotted ones that look like a rat is living in their hair, and you keep expecting to see bugs crawling out of it. They have it because they're dirty, nasty, hippies.
Personally, other than the last kind, I think they can be hot as hell on the right person. I find them way less offensive than pasty legs in shorts.