Why was control of the seas so important for the Allies and the Axis?

Answer 1

Goods were shipped long distance across the oceans and seas. These goods included raw materials, petroleum, military personnel, food, and armaments.

Answer 2

The Americans (until they entered the war) remained neutral. So the British were responsible for getting their own supplies. The easiest and only feasible way at the time was by sea. This was why the Germans attacked British ships(to prevent the British from receiving supplies, food, guns,etc).