What the reason for using the atomic bomb?

The reason for using the Atomic Bomb in WWII was to prevent an invasion of the Japanese homeland. If the U.S. would have invaded the Japanese homeland there would have been more casualties both American and Japanese and WWII would have lasted longer. People say the U.S. should have never have dropped Atomic Bombs because the Japanese didn't drop A-Bombs on them, but the Japanese would have if they could have.