Like we did in Hiroshima? Tell me there was a reason, other than
vindictive, hateful, annihilation.

The reason was twofold:
(A) Scientific. It was a safe way to test a technology on a civilian population
with little ethical restraint because the testers were locked in a desperate
war. The atomic bomb had no strategic military relevance at this point, since
the war's outcome was certain.
(B) Political. The US knew that as soon as the Axis was dispatched it would be
facing the Soviet Union & the atomic bomb was seen as an important means of
telling the Soviets to back off.

Scott Fraser