If nobody had nuclear weapons the second world war would have lasted longer and the invasion of the Japanese mainland would have cost far more lives (on all sides) than the bombings of Hiroshima and Nagasaki did. The cold war would probably have gone hot, Europe would have been a battlefield and might have ended up under Soviet rule. As to how the conflict between China and the Soviet Union would have gone I can only speculate, that there would have been one is rather likely.

Nuclear weapons have kept the large powers from direct confrontation since the 50's, their existence has saved more lives than their use has cost. As long as they are not acquired by (in our eyes) irrational or suicidal actors they can be used for this purpose. As to whether a nuclear armed Iran would use them as a deterrent or otherwise - which includes providing nuclear weapons to third parties for use against common enemies - I can only guess. My guess is that a weakened theocracy - which is what Iran is - is an irrational actor and with that not to be trusted to "do the right thing".