Raising the overall costs to the US and its allies is a pretty coherent theory of victory for Iran. Obviously they aren't going to win a conventional fight, but they might be able to inflict enough havoc on energy and commodity markets to the point that it really hurts the US and its allies economically; perhaps enough that they bail out of the war in order to stabilize the global economy.
Trump clearly wanted a quick easy win here and does not want to see massive inflation at home. Sure he personally doesn't give a shit about Americans but the rest of the politicians who enable him do and he's at risk of absolutely torching his own party for years if the war drags on and costs really get out of hand.
All the Iranian regime has to do to win is not lose for enough weeks. If the regime holds out Trump will have to either give up and try to pretend this disaster was a Great Victory, or he'll launch a ground invasion that will almost certainly turn into a quagmire. Bombing civilians makes a popular uprising much less likely, so the US is doing them quite a favor on that front.
It's their only play, really.
Iran has deliberately escalated the war horizontally to create a bigger mess and to make the military adventure more expensive for America and the world.
Iran is saying, "If you attack us, these are the costs."
As an invading military, you're either willing to pay those costs or you're not.
They are easier to hit and harder to defend then Israel. That is depleting defense forces more.