After being chased out of Boston, the British army was looking for a good place to reenter the colonies. The British had the world's largest and best navy. The colonists had no navy. So a place made up of islands and rivers would seem to give the advantage to the British.
They decided on New York.
New York was chosen because of its large harbor, all of the rivers in and around it, and all of the islands that make up New York City.
Washington's spies told him of the British plans so he rushed his army to New York and prepared to defend it. Washington tried four times to drive the British out of New York City, but to no avail. Washington finally gave up, and abandoned the city.
The loss of the largest colonial city, New York City, was a major blow for the colonists because New York City offered:
A major port for the British to get supplies
A centralized location in the colonies
Easy river access to the interior
The loss of New York and the defeat of General Washington at the battles of Long Island, Harlem Heights, White Plains, and Fort Washington allowed the British to do pretty much whatever they wanted to do next. So they set their sights on Philadelphia, the Capital of the 13 colonies and the home of the Continental Congress and Independence Hall.
As the British marched toward Philadelphia, Washington and his defeated army could only watch. The Continental Congress had to flee Philadelphia, moving away from the advancing British.
The British felt very confident about victory and an end to the rebellion. With winter starting the British army settled into Philadelphia to wait for spring to beat those pesky Colonials.