This question has been thoroughly discussed since, oh, their first development.  Nations hold onto significant stockpiles of nuclear weapons not for their destructive yield, but to deter pre-emptive first strikes.  With nuclear weapons placed on submarines, on mobile platforms, and in shielded locations around the nation, any opposing nation would be forced to doubt their ability to eliminate all these weapons in a first strike with the knowledge that the launch of just one of these would resort in the loss of millions of lives in their own homeland.  The success of deterrence continues to depend upon the logic of Mutually Assured Destruction. Those unwilling to accept this logic are hopelessly naive about the benevolent nature of mankind.