I think he's there for a backup, and like M said, some wildcat formation, maybe a slot back. He's there to give the other team that much more to prepare for defensively. He was never a very accurate passer, and was more dangerous running the ball than throwing. He's getting older, and has been away from the game for a while, so the running may not be what it once was. Still, they probably got him cheap, got an experienced backyup and the bonus of making it harder for the opponent to get ready for game day.
I would expect Philly is more the sort of place where people will take the attitude that he's served his time, and won't fault their club as much for the signing. They'll still bark at him though.
Gotta love Philly, the parking lot goons always used to let us park in the press parking area for a $20.