The rings allow the hub to be used as a centering point, so that when you place the wheel on the wheel hub, the wheel rests tightly on the hub, instead of hanging on the wheel studs. If you were to install the lugnuts without hubcentric rings using an impact gun and not following a star pattern, the wheel may not be perfectly centered, which can result in vibration. I've had good luck so far without rings, but I always hand tighten the lugs in a star pattern when the car is in the air, then hand tighten again once the car is on the ground (don't have a torque wrench yet). Hubcentric rings are not load-bearing once the wheel is secured to the hub with the lug nuts.
Basically it's better to have them. They're cheap anyway. Use metal rings whenever possible, I've heard of plastic rings melting.