Submarines were used during World War 2 for what purpose?

submarines were widely used by both sides as the ultimate weapon of naval blockade, sinking large numbers of both merchant ships and warships, resulting in either paralyzing the enemy's military industry and war effort by causing severe shortages of war materials and products and preventing maritime troop transfers, or forcing the enemy to dedicate enormous resources to anti submarine warfare in order to prevent that from happening, resources which could otherwise be used in the enemy's main war effort.