Why did German U boats sink unarmed ships?

German U-boats sank unarmed ships during World War I and II primarily to disrupt enemy supply lines and weaken the opponent's war effort. The practice was part of unrestricted submarine warfare, aimed at blockading and cutting off supplies to Allied nations. German commanders believed that by targeting both military and civilian vessels, they could instill fear and exert pressure on enemy economies, thus hastening victory. This strategy, however, led to significant international backlash and contributed to changes in naval warfare rules.