After winning the Battle of Brandywine, the British captured Philadelphia on September 26, 1777. The British army finally left the Continental capital almost nine months later on June 18, 1778, following France's entry into the war.
Howe was defeated early, so he wanted an easy victory and he thought that Philadelphia would be an easy win for him. It had little strategical value and his orders weren't specifically to take it, but it was how he decided to interpret in order to erase the stain of a defeat. Or so he wished that it would have been the case.
