Funny I should see this thread today, since I saw a guy doing that after I got off the train this evening. He was a tall kid and his pants were sagging so low, his entire backside would be exposed had he not had boxers on. Every other step he had to pull his pants up to keep them from falling all the way down.
There was a guy I worked with who did the same thing. I asked once why he did it and he couldn't really give me an answer aside from "it's comfortable". Myself, I've never liked the feeling of my pants about to fall down.
I will relate a story I heard from some movers last year. One guy was telling his buddies that kids today don't know what the sagging really means. He said that in the "old days" jail inmates would sag their pants as an invitation for other inmates to have sex. Whether this is true or not, I don't know. But that's what he said.