2 reasons I can see
There are 2 reasons why Philadelphia would be a strategic point in when it comes to the grand scheme of things.
A) Strategic location: Britain could very easily ship things to and fro from London to Philadelphia, it can serve as a sort of supply depot, much like NYC, it is a place where you can fall back to if need be, and it is certainly easy to defend.
B) Moral importance: As you know, Philadelphia was the site of the Second Continental Congress, it would deal a huge moral blow if the city where the nation took its "baby steps" would be captured and ransacked. The declaration of independence was (of course) also signed here.