I was going to explain it but found this:

Hub centric rings are manufactured so that when they are installed on a vehicle, the wheels are located directly off of the centre flange of the brake drum or rotor. This is done to insure that the outer beads of the wheel are concentric with the wheel bearings. The result is a much truer, better balanced tire/wheel assembly. This is very important with modern high tech suspension systems.

There you go! Throw them on and then the wheels.....