Answer:
This is a very opinionated and debated question.
The Common Theory (US Perspective):
The bomb had to be dropped on the cities in order to prevent the prolonging of war and death. More casualties would happen on both sides, Japanese and American.
Counterargument:
The bombs should not have been dropped because the Japanese were going to surrender soon. This was due to the USSR invading Japanese occupied China, which the Japanese did not want to be occupied by the Soviets. So either way, Japan would most likely have surrendered in 1945 on American terms due to the fear of Soviet occupation.
Other Arguments:
The bomb was used to show off American military might since WW2 was ending the Cold War between the US and USSR was beginning. The atom bombs did scare the Soviets when they were used on Japan.