same reason why some people wear rings in there ears and nose and (?????????[]?????????).other places.what you like
just a personal prefference.also some dogs work better(Tail gets Bloody) some are for the breed (bull dogs) because they use to fight them all around,another dog would catch by the tail.depends on the breed and it's history.

some dock at the base(1 joint) others two joints ; others dock 3/4?????