Nations (and people) don't always act in their self interest. It doesn't matter WHY Iran wants the bomb it only matters IF they have the bomb. Possessing nukes is a game changer for a number of reasons. Using a nuke ANYWHERE would cause the West to collectively lose its mind and bomb, bomb, bomb. Iran would cease to exist and even a conventional response would result a decent amount of Persian population control. Let's be honest, Syria's use of WMD didn't cause any great alarm, but a nuke? That's Pandora's box. Benefit from using a nuke? Land would be a buyer's market in the deploying country.
I don't think Iran wants a nuke to use, it wants a nuke for deterrence, pride, and dick measuring.