I own a variety of guns because they are different or fill a specific niche, but I honestly don't understand the need to own or collect multiple ARs when they are all basically the same.
Aside from owning AR's in different calibers, there are a variety of reasons one might want to own several AR's in the same caliber. For example, different barrel profiles and twist rates allow the use of different bullet sizes. I've got a 1:7 heavy barrel I use for 68-77 grain bullets from the bench, but I've looked at getting a 1:10 or 1:12 in a similar profile that would be much better suited for the lightweight varmint bullets. Then beyond the heavy barrels I have a Rock
River National Match that I use for shooting open sights in service matches, and then also a couple of carbine length rifles that I use for sport, competition, and defensive shooting.
Another thing that's nice about multiple AR's is the option of running totally different optic setups from fixed open sights on the National
Match to reflex sights, 1x4's, and BUIS on the carbine sized guns to variable power scopes and no BUIS on the varmint and benchrest guns. So there are lots of reasons to have different AR's that are not at all the same. Another really nice thing about this too is that you then have guns configured in different ways for different needs, but it's all one platform on which you can train and become especially proficient. This is why I love the AR platform so much!