How role did submarines play in World War 1?

In World War I, the Germans used submarines mainly to create a Submarine Blockade on Britain in an attempt to starve Britain out of the war. This blockade is similar to the Naval Blockade Britain used on Germany, but was not as effective, and Germany stopped halfway through the war. However, towards the end of the war when Germany was losing, they restarted the 'unrestricted submarine blockade', which accidentally sunk an American passenger ship. This angered America, leading to America's participation in the war, and the ultimate defeat of Germany.