It was in no way necessary for the United States to use the atomic bomb on Japan at the end of WWII. There were much better options available. The Russians were willing to help fight Japan, and the use of the atomic bomb would hurt the United States foreign relations more in the long run.

The Soviet Union had already agreed to aid in the war against Japan in return for some pieces of land. During the conference at Yalta, Roosevelt promised Stalin the Kurile Islands, the Southern part of Sakhalin and various railroad facilities throughout Asia. Stalin agreed to these terms. The United States most likely could have convinced the Soviets to do all the dirty work, and invade and put down the Japanese entirely by themselves by simply promising them even more land. Even without make additional promises to the Russians, they had already agreed to fight. The United States could have waited just enough time so that the Japanese wasted all their suicide bombers on the Russian army, then invaded the country which was already severely weakened. The United States had already recaptured most of the Pacific islands that Japan had taken over and consequently destroyed most of the Japanese military force in the process. The use of the atomic bomb also would throw the Soviet Union into an arms race with the United States, which would later result in the Cold War.

It was most definitely not necessary for the Untied States to use the atomic bomb. They could have used the Russians to weaken Japan instead, and even kept the Russians happy be giving them some more land.