What the US has often done in the past is provide air support for aligned local rebels on the ground. In this case, though, that rebel force doesn’t really exist, so I figure the reasoning is to show the people of Iran that if you want to take back your country, now’s the time - and we’ll help you. There’s at least some evidence that a big chunk of the Iranian population is fed up with the Islamist government but whether this’ll lead to an uprising or regime change remains to be seen.