Why was the British Navy essential to any British victory in America?

The American Revolutionary War saw a series of military manoeuvres and battles involving naval forces of the British Royal Navy and the Continental Navy from 1775, and of the French Navy from 1778 onwards. These culminated in the surrender of the British Army force of Lieutenant-General Earl Charles Cornwallis, an event that led directly to the beginning of serious peace negotiations and the eventual end of the war. From the start of the hostilities, the British North American Station under Vice-Admiral Samuel Graves blockaded the major colonial ports and carried raids against patriot communities. Colonial forces could do little to stop these developments due to British naval supremacy. In 1777, colonial privateers made raids into British waters capturing merchant ships, which they took into French and Spanish ports, although both were officially neutral. Seeking to challenge Britain, France signed two treaties with America in February 1778, but stopped short of declaring war on Britain. The risk of a French invasion forced the British to concentrate its forces in the English Channel, leaving its forces in North America vulnerable to attacks.