Some people deliberately try to get banned. I'm glad to oblige them. They sometimes attempt to return under another username, but it's pretty easy to see a pattern emerge. If you ban someone once, do you need to explain why you're banning them again? I don't think so.
Some people do something wrong without realizing it and get banned. We generally hash it out over email and they get a second chance.
Some people make a big stink about getting banned. Trouble follows them wherever they go. They revive old arguments, some of which occurred years ago on another site(!) and stir up the stink. They're banned multiple times, appealing each time, often to my coworkers. But it's clear they don't learn their lessons and as long as they're around, trouble will be close behind. They don't understand how to act on a message board and refuse to follow good, genuine advice from well meaning peers. These people are perhaps far more dangerous than the most blatantly offensive posters.
Banning those people is often doing them a favor.