In effect, and to the abused, absolutely nothing.

For those treating patients with NPD, the diagnosis provides a framework for understanding and treatment.

Most with the disorder will not seek treatment, as they don't think they have a problem.

Thinking of it as a disorder might help taking rational actions to fight it, like any sickness, eg get away from the person, understanding it's not your fault, and maybe get them medical attention etc

Also, consider that many psychiatrists and psychologists believed that the 45th president had this disorder. Some who worked with him (Paul Ryan, for example) were convinced as well.

Of course, diagnosis from afar is fraught, as is diagnosis by non professionals. But I think the former president provides a good frame of reference for how people with NPD behave, but taken to an almost surreal extreme.

NPD is largely defined by the inability to experience empathy. They don’t know they are assholes, or worse, feed of a high they get when people think they are assholes when they think they are right.

CAD - colossal asshole disorder