Why U-boat is important?

U-Boats were important to Germany, because that's the only navy they really had. They had some surface vessels, but they amounted to just about "token" forces. Even though the British considered them abit of a challenge in the beginning, they really meant little to the US Navy; of which the German surface elements were neutralized prior to Pearl Harbor anyway (Dec '41).

Germany had to fight a "Guerre de Course" (sinking supply ships) because they possessed no battle fleets; neither battleship nor aircraft carrier in WWII.

Had Germany possessed a first rate battle fleet, they could have avoided a merchant ship war and clashed on the high seas in the traditional way. But they didn't have one, thus the importance of their submarines.