There are a few possibilities.
1) She’s staying in the race to be in as strong a position as possible so that Obama has to pick her for vice president. This is the only rational explanation I can think of for her behavior, given that she is not an issue-driven candidate, pushing an agenda different from Obama’s. When she finishes out the primary season with 18 or 19 million votes cast for her, it would be surpassingly arrogant and possibly a little demented for Obama not to choose her. Forget all the nonsense about how Obama represents a fresh start and Hillary would muddy that message. Between them they will have had 40 million votes cast, and if they could count on most of those votes in November, they are two-thirds of the way to a victory right there. (Though, God knows, Bill is a problem; how could Obama be sure he would keep his mouth shut and not damage his chances in November? That will have to be closely negotiated. Or maybe he will have to go into a medically induced coma for a few months.)
2) She’s hoping that something is going to bring him down between now and the convention. Can’t imagine what that would be, unless Obama’s crony Rezko is convicted and, in a desperate effort to save himself from jail, does something that destroys Obama’s career — though it’s hard to see what good that would do him.
3) She’s trying to show Bill she’s as tough as he is. My guess is this is inarguably true, but it doesn’t vitiate point #1.