Question

What were the reasons the bomb should have been used on japan?

Answer 1

The short answer is that it was used to end the war early and save US lives. Military planners were convinced by the evidence on hand that an invasion of Japan would have been incredibly costly in terms of casualties after seeing how most Japanese soldiers would often choose to die to the last in battles from Okinawa, Iwo Jima, the Philippines, and Saipan.

At the same time, no one knew the full effect of what an atom bomb did to the human body. Radiation sickness, fallout, etc.. were little understood at the time. Although using it to see just what would happen wasn't a military priority, studying the horrific after effects did educate the military on what using such a weapon led to.