They're useful for:


1. Long distance shooting.
2. Competition	
3. Practice
4. Plinking/recreational shooting
5. Varmint hunting
6. Big game hunting, in the proper caliber and legal magazine.
7. Self-defense.


Sure, there are other guns that can do any of these better, but an AR is the only gun that can do them all. You can go from a self-defense gun chambered in .300 Blackout with low penetration rounds to an upper with a 16" or 18" barrel for competition to a 24" long range shooting barrelled upper in about 10 seconds per change. 


We have 13 million ARs and AKs in civilian hands with hundred of millions of 30 round magazines. Less than 300 people are killed each year with any kind of rifle, and handguns and shotguns are easy substitutes for a rifle in the close up killing we see from these nutcases.