Biden is a spoiler, he's there to take the heat off of the other candidates. He's an easy target and has always been known as a gaffe-machine, he's run for president and failed more than once.

He does well with african-americans, women, and centrist/status-quo dems, where some of the other candidates don't.

He'll drop out for medical reasons (probably real ones!) before the nomination and endorse whatever centrist dem is polling best.

Remember what Obama said, "Joe, you don't have to do this." Biden views it as a form of patriotism to protect the other dem candidates by being the big target.