Why did the british occupy philadelphia?

  On September 11, 1777 British General William Howe's army defeated George Washington's army defending Philadelphia at the Battle of Brandywine. Washington and his army were forced to retreat, there was no longer an American army defending Philadelphia. The British marched in and occupied Philadelphia...until the fortunes of war changed and the British left Philadelphia in June, 1778 without a fight.

Howe's campaign was controversial because, although he successfully captured the American capital of Philadelphia, he proceeded slowly and did not aid the concurrent campaign of John Burgoyne further north, which ended in disaster at Saratoga for the British, and brought France into the war.