Hub centric rings are also known as centre bore locating rings. Producing one wheel/rim to suit each hub size would result in a lot of waste and take up a lot of inventory space. These rings are used to adapt aftermarket wheels to suit your hubs, ensuring the wheel is mounted in the centre relative to the hub, avoiding vibration and instability.

Ring sizes explained:
The first number is the outer diameter (ie. the centre bore of your rim), and the second number is the hub diameter on your vehicle (or you can also measure the centre bore of your factory rim).