It is multi-faceted, and some of the facets aren't just about his on-field role.
(1) On the field, his role is kinda like what Slash's was. He can come in as quarterback for a few plays in some alternative formations. Maybe he spreads out wide for a play or two. Maybe he serves as a distraction for a play or two. It is one thing to take off after scrambling when everyone on the Di has their back to you or has to chase you. It is another to come across the middle and they are waiting for you.
(2) As crazy as it sounds, he serves a marketing role. He creates interest - good and bad - and ultimately, in this situation, I believe that might equate to increased revenue and marketing prominence for Philadelphia within the NFL. If Vick pulls off the 'I repent' gig I can see that. If he starts getting in trouble...
(3) He actually takes a little pressure and focus off of Donovan. Donovan won't have to deal with as much of the 'should Donovan be our quarterback' talk - at least not early in the season. When was the last time that wasn't the primary focus of the football conversation in Philly? Personally, I think this part of it may backfire, because I see McNabb as a prima donna who doesn't like it when the focus isn't on him. To me, he puts more pressure on Donovan every time he misses a receiver or gets picked
(4) Here is the gem for Philadelphia. He has the potential to become a trade-able football asset for Philly, if he gets through this year from a PR standpoint. He gets through the worst of the public outrage over him playing. He gets back into football shape. He makes a few plays on the field and shows that he still has the ability to be a dangerous weapon. Philadelphia took the chance and dealt with the potential media circus, and in so doing they redeemed his value as a player. Now, they have an option for next year, and the rights to a Vick that a lot of other NFL teams may have an interest in. Perhaps, for their small salary investment this year, they could get a 2nd or 3rd round draft pick from someone for him next year.