What possible use are nukes for Iran anyway?
Merely possessing nukes changes the power dynamics. You don't have to use them to alter how your neighbors, trading partners, and enemies perceive you. Even if you have no rational way to use them, you need only convince people that you're capable of behaving irrationality to gain leverage.
Take Israel... they've done some pretty crazy stuff, and they maintain deliberate ambiguity over whether they possess nukes or not. If you're Iran, it might be nice to have a back-pocket action there. Of course, they're probably also aware that they sit b/t Turkey (who hosts US nukes), Russia (their love/hate trading partner across the Caspian Sea), and Pakistan. And wouldn't it be nice to intervene diplomatically if things got heated b/t India and Pakistan, since fallout doesn't confine itself to national boundaries? And while Saudi Arabia doesn't have nukes, anything they can do to one-up them and gain more influence over the region is highly desirable.
Basically, nukes put you in a very exclusive club.