This is not a good question to ask. It's not for what. It's to prevent what.

Even if you don't care about how Iran treats its own population, you should care about how it treats others because one day it might be you.

Iran's ideology is expansionalist. They want to export their ideas far and wide. While also physically suppressing their opponents.

Since the emergence of two global superpowers after WW2, most armed conflicts were, essentially, between those two. Directly or indirectly. If the two could find a peaceful solution, the world at large had a good day. And for a while it seemed like we might be left with just one superpower, and so no conflicts at all. Only policing.

The whole "multipolar" spiel you might hear from powers that challenge this policing is what is going to result in a long and bloody struggle for world domination. It won't be like the world wars we remember, where most of the world had to choose one of the two sides. It will be a quagmire of more local conflicts with very volatile alliances and a lot more casualties overall. More like Italian city states.

It's necessary to police Iran and similar bad countries because if their challenge of the power monopoly succeeds, the world will be hurled into anarchy equipped with very deadly weapons...