What is the U-boat used for?

Germany originally built submarines for export and sold some to Russia in 1904 during the Russo-Japanese War (1904-1905).

Germany's first U-boat (U1) was taken from the Russian batch and commissioned into the German Navy in 1906.

German U-boats are simply submarines; German's call them "under-sea boats." Subs are used for sinking ships or reconnaissance (scouting) work.