@Phoenix32890,
Why?
Seriously...
Obama is himself the change agent. He has said consistently that the way to change is to work -- effectively -- within the system.
Biden entered Washington as a change agent and has been someone who Gets Things Done during his time there. He's someone who knows how to work effectively within the system.
"Effectively" is key here. Hillary Clinton, for example, was extremely ineffective in her approach to health care, and Obama has said he wants to do several things differently. (Keep it open, not behind closed doors -- working with people, not just demanding capitulation... etc.)
Ultimately, as you say, Obama would be in charge. Biden seems genuinely fine with that -- he knows that his own chances for becoming president have dwindled to near-nonexistent, and he genuinely admires Obama and seems excited about this opportunity.