But if we are truly deterministic beings, the factors determining our environment are incredibly important. Even (not freely) acknowledging that free will doesn't exist we could very well (not freely) decide that we need a justice system in this society because we (not freely) want less crime, and people will (not freely) do less crime in a society where such a system is in place.

In the end it doesnt matter if people act based on free will or entirely predetermined. Or society developed as we are, and we put systems into place that seem to work. Sure, someone robbing a bank might do so for reasons that were predetermined in his brain and surroundings, but getting prosecuted for it would in turn become something that codetermines every future moment of his life.

The only think determinism really changes is perspective. It enables us to say: Okay I understand why I/they/you acted this way, or maybe I don't understand, but can assume that there were reasons. That's it. It lends understanding; it doesn't have to chance anything.