What's the point of CD drive offsets?

From AccurateRip's site.

Each CD drive reads audio discs slightly out (a number of samples), if your CD drive supports 'Accurate Stream' it will be a constant value, this value tends to be the same for each particular make and model of CD Drive. A small number of drives have [Purged] as the offset, these drives were found not to have a constant drive offset (perhaps different manufacturing batches, or firmwares), as such they have been removed from AccurateRip's drive database (should you have one of these drives, 3 matching key disks will be required to configure AccurateRip).

The page contains a list of known Drive's in AccurateRip's database and their respective offsets.