They're basically so you can ride your horse in a halter a little bit easier, since you can just clip the reins to those rings on the nose instead of tying the reins on or whatever people do.
Personally, I'm not a huge fan of those. I have one that I made were the rings are halfway between the nose knots and the chin knot which I like pretty well. I feel like having the rings on the nose knots like that encourages the nose to slip around and be less reliable. It doesn't slip if the rings are farther down, it's just more work to make one with the rings farther down.
Same as Wallaby said: It's another bitless option. The knots on the noseband provide a bit more bite than the average halter. I don't mind these, but they do tend to slide around. I ride my mare in the noseband of one of those attached to a normal headstall.