They dropped two nuclear bombs on Japan to make the Soviet Union's invasion of Hokkaido unnecessary and as a performative act to horrify the world in to subjugation.
The whole 'saved more lives than it cost' is a bad argument unless the plan was to slaughter a hundred thousand civilians on landing. Note the Nazi invasion of France cost 60,000 lives in civilian resistance...this was at the beginning of the war.
Japan would have surrendered on the first day of invasion.
I like Dan Carlin's podcasts, but his most recent Hardcore History series focuses on just this very thing. Between that and his constant political "both-siderism", I'm probably going to stop listening to him.
Carlin is an absolute coward hack who hides behind the flimsy excuse of "I'm not a historian".