Control of the shipping lanes and unimpeded access to the entire coastline of the colonies gave the British a serious strategic and tactical advantage over the colonials. The British Army could resupply at will with food, ammunition, clothing, artillery, and anything else it needed. It could prevent trade from the colonies to anywhere else, seize merchant ships, and interdict supply lines from the French later in the war.
Even more importantly, their military had mobility the colonials could only dream of. In the battle for New York City, for example, the British could land anywhere from Long Island to Brooklyn and Washington had no idea where, nor any means to effectively react to it. Reinforcements could be moved up and down the coast without challenge to wherever they were most needed, and units that ran into trouble could be evacuated or resupplied by sea.
This is why the French Navy's appearance at Yorktown was especially important. It negated one of Britain's most important advantages up to that point, and proved decisive.