The sanctions have real, tangible practical effects. It's not just about making them suffer. For example, Hezbollah who are almost exclusively funded by Iran, had their budget cut by half [0]. The military operations Iran undertakes in Syria which are intended to strategically threaten Israel have taken a cut. It's hard to justify supporting proxy wars when the population at home is hungry.

[0] https://www.thedailybeast.com/as-iran-us-tensions-rise-with-...