Offset is just a different way to measure the rim specs. If you have a backspace measurement, you have an offset measurement too. Backspace is measured from the lip of the rim to the mounting surface and is usually listed in inches. Offset is measured from the centerline of the rim to the mounting surface and is listed in mm. Both are just easy to use.
Offset is easier to use if you’re changing or comparing different rim widths. Offset tells you the centerline of the wheel. If you have your wheels centered in the wheel well, and want to use a wider or narrower rim, you just keep the offset the same. If you keep the backspace the same but change the width of the rim you’re actually moving the rim in or out depending on the change in width.
