This is a move to win PA... Romney already has Central and Western PA. He needs the Philadelphia suburbs (many socially-liberal fiscal conservatives) to turn toward him, and the best way to reach them is an ad buy in Philadelphia.
The Philadelphia TV and radio stations reach a pretty broad area - the Philly suburbs, central PA, northeastern PA (including the Pocono Mountain area), southern New Jersey (south Jersey leans more Republican than north Jersey), and most of Delaware. Hopefully the ads will allow Mitt to pull in voters in the city of Philadelphia itself, as well as in Delaware and New Jersey.