What are centric rings needed for?

Hub Centric Rings are used when the hub hole (the center bore) on the rim is larger than the wheel hub of the vehicle. Wheel manufacturers deliberately make the center bore on the wheel as large as possible,  for the many variations of other vehicles the wheel can fit so the same wheels can be sold for the various models of cars using hub-centric rings.

Centric rings are usually needed only for custom Wheels since the original wheel usually comes hub centric (the Center bore and hub of the brake rotor or drum are the same size ). Usually, the hub-centric rings are used with alloy wheels, but sometimes steel rims may need hub-centric rings. The purpose of the centric ring is to perfectly center the alloy wheel to the wheel hub and hence prevent vibration to the steering wheel during driving and the shaking which typically appears around speeds of 40-60 mph. It is the same type of issue when you have too many clothes on one side of the washing machine.