Saving American Lives and reducing cost of rebuilding Japan.
Step-by-step explanation:
Truly the US wanted an excuse to use a new weapon. many were against the use of the atomic bomb in the wat, but the argument that decided the fate was one of calculation in time, money, and lives.
With the Atomic weapons used, the war would be ended several years earlier rather than completely invading Japan where the most hostile front in the pacific would have been (the "hornets' nest" so to speak). This would have also accomplished saving potentially hundreds of thousands or even millions of US service members lives.
Lastly the cost, the US held no intention of defeating Japan and then leaving the nation to do what it pleased. So if they were to occupy and begin a war of "winning hearts" it made it much easier to do when the US only had to rebuild two cities as opposed to the billions of dollars it would take perhaps trillions; to rebuild the nation entirely.