Most people would answer that the USA used nuclear weapons twice: Hiroshima and Nagasaki.
Well, the USA used nuclear weapons many times, without exploding them. For instance, we told the Soviet Union that if their tanks invaded West Germany, Moscow would disappear. That is using nuclear weapons without exploding them.
In many ways it is more effective to use a nuclear weapon by not exploding it than by creating a mushroom cloud. When you create a mushroom cloud, the enemies fears have been realized and they have nothing more to fear from you, so they will through everything they have at you. An unexploded nuclear weapon can also be used many times.
Of course, you have to have nuclear weapons in order to be able to use them (either as a deterrent or to punish an enemy). That is the main reason we need to maintain our nuclear weapons systems.