Not all Doberman Pinscher dogs get their tail cut but the ones that do get it done by the breeders and owners due to fashion and the way they think the dog should look.
It's just what most people want done when they get a Doberman Pinscher although I think it's cruel and shouldn't be done and even the American Vet medical association doesn't agree with cutting the Dobermans Tails but it's still done.