The threat of using a nuclear weapon is far more potent than actually using one, primarily because every world leader in a nuke-possessing country is keenly aware that if they push the big red button, they will instantly become a global pariah and even their staunchest allies will turn against them if only to save their own hides from the inevitable response.
But if you keep hovering your finger over that button and going "Oh! Oh, am I gonna push it? Am I gonna do it, eh? Am I really THAT crazy? You suuuuuuure you wanna find out?", you're holding much more power because nobody really does want to find out.