The ring serves 3 functions as I understand.

-Tracking of the controller: There are leds on it that emmit IR light so the tracker can accurately track the controller. This accuracy is not achievable at the moment with just internal sensors (although the controller has these, presumably for even better accuracy)

-The inside of the ring has sensors to track your finger gestures. I can imagine this would not be possible with just the internals of the controller.

-The ring acts as a sort of extra handle. Imagine opening your hand totally (as in waving). This is a gesture you would maybe like to do in VR. The ring will hang between your thumb and index finger while you do this. Other more traditional tracked controllers would fall out of your hand.

A target with a wide baseline for the tracker.

Protection from hitting your hand while swinging the controller around?

They mentioned finger gesture detection (thumbs up, pointing, etc). Maybe that's part of it?

Looks like they may have IR LED's around the outside that are used by the camera for positional tracking. Might also be used to track finger movements on the inside of the ring, but not sure there.