What are nuclear bombs good for?

Why did Japan surrender? Very interesting---the scholarly claim reported is that Japan surrendered only because of the Soviet Union's entry into the war deprecated their strategy to try "to convince the Soviet Union, still neutral in the Asian theater, to mediate a settlement with the Americans. Stalin, they calculated, might negotiate more favorable terms in exchange for territory in Asia." Moreover:

The bomb - horrific as it was - was not as special as Americans have always imagined... The photos of charred Tokyo and charred Hiroshima are indistinguishable. In fact, more than 60 of Japan’s cities had been substantially destroyed by the time of the Hiroshima attack.

However, the journalist speculates a bit too carelessly at the end:

Hasegawa’s scholarship disturbs this simple logic. If the atomic bomb alone could not compel the Japanese to submit, then perhaps the nuclear deterrent is not as strong as it seems. In fact, Wilson argues, history suggests that leveling population centers, by whatever method, does not force surrender... If killing large numbers of civilians does not have a military impact, then what, Wilson asks, is the purpose of keeping nuclear weapons? We know they are dangerous. If they turn out not to be strategically effective, then nuclear weapons are not trump cards, but time bombs beneath our feet.

Sometime between 1945 and 1961, the stakes increased from the destruction of a few major cities to the destruction of all major cities. Mad or not, MAD worked: the Soviets withdrew their missiles from Cuba and Kennedy secretly agreed to withdraw US missiles from Turkey. Moreover, in a counterfactual 1945 in which Japan didn't surrender and we kept dropping nukes, the eventual result would have been a sparsely populated, poisoned Japan. At that point, surrender would be irrelevant.