I go by other usernames on other sites, but never alts on the same site... On other sites, it is simply because I have a few normal usernames for different things, just depends on what I feel like when registering... GIR7590, Dopefish7590, and Angelfire7590 are my names, but Dopefish7590 is my main name.
But I know people who use them to avoid association with something they like. D: