They enabled a relatively weak naval power (Imperial/Nazi Germany) to potentially neutralize the sea advantage of a much stronger naval power (British Empire). The primary characteristic of the submarine as a weapon is it is extremely hard to detect and intercept before it attacks. read more
Because they were not only part of the US entry into the war, but also a devastating new weapon. The first attempt to create a armed U-boat was a devastating move, for the Germans. One U-Boat hit a landmine. read more