This question is not really one you can answer. It was a complicated series of factors and influences that led to the series of events known as the Revolution.

That being said, Philadelphia's importance was that which was assigned to it by the continental congress (as far as I know). The congress could have met in New York, Princeton, Harrisburg, etc. and there is a very good chance the war would have ended up the same. The events of Boston, however, were more due to the city's culture and status. I don't want to speculate as to alternative history too much, but events might have been prolonged or happened completely differently if not for Boston