Iran has repeatedly denied that it is pursuing nuclear weapons and even declared them to be un-Islamic. For a long time I, like many other people, responded with, so what. Who cares what the Iranians say if they are lying?
But then others pointed out the answer. Even if the Iranians are lying, their declarations matter. Why? Because the declarations mean that even if they are lying, the Iranians can adopt a course of action that prevents them from getting nuclear weapons without losing face. They can simply claim that they never wanted nukes in the first place.