Does anyone know the original logic behind the practice of tail-docking or not of similar purpose breeds?

I am specifically interested in the reasoning here, not whether or not you think this is a pointless or cruel practice.
For example, why dock the tails of Norfolk Terriers but not West Highland Whites? Or, the tails of Australian Shepherds but not Border Collies? German Shorthair Pointers but not Pointers? Some breeds of spaniels but not others?
The comparisons I cite are of breeds that have such similar purposes that I can’t figure out what function a docked tail would serve.
Thanks.