[sesWaipunsaws]

From one of the many articles on the subject:

The Romans docked tails in their ignorant belief that the procedure prevented the rabies disease from occurring. Herding breeds tails were docked because of the nuisance of burrs and fox tails getting entangled. Others were docked because of hygienic reasons especially in those dogs with lots of hair around their tails.

Dog breeds that were historically used as working dogs appeared to have encountered various issues from their tails getting severely cut as they ran through bushes. Sooner or later infections would set in making tail docking a common preventative procedure. With little or none veterinary at those times, docking seemed like a wise preventive measure.
It must be considered as well that centuries ago in Germany, taxes were collected from the owners of non-working dogs, these taxes were defined as "tail taxes" since the distinguishing feature between a working dog and a non working dog was the presence or abscense of it's tail.
This is because most breeds in particular had their tails docked depending on how they were primarily utilized at work.