Why were submarines significant in World War 1?

Submarines were significant in World War I because they inflicted a lot of damage on shipping around Britain. They sank both military ships, supply ships, and anything else, which brought out the term unrestricted submarine warfare. To site a single example, the U-9 sunk three British cruisers within an hour. We also saw the strategy of the convoy appear in defense. The other significance of submarine warfare in World War I is that the capability of the submarine was clearly demonstrated, and the strategy and tactics (as well as the technology) was developed even further. World War II submarine activity was an extension of the lessons provided in the first war.