In relation to this question Are there any behavioral or health issues related to docked dogs?.
Apart from cosmetic reasons, what reasons are there to dock a dog's tail?
1 Answer
From AMVA FAQ
Removal of a dog’s tail for medical reasons is not referred to as “docking.” The most common reason for amputation or partial amputation of a dog’s tail is traumatic injury where repair of the entire tail is not possible or advisable. Amputation may also occur in the case of tail deformities that negatively impact a dog’s function or increase risk of injury. An argument might be made for removal of the tail of a dog on the basis of repeated prior injury.
So there are times where a dogs tail will be removed for medical reasons.
Precautionary removal of the tail of a young puppy needs to be based on compelling evidence that the animal is at high risk of tail trauma due to congenital defect, breed and/or planned working activity. However, such a justification must be supported by evidence such as empirical data or impartial expert opinion based on extensive, directly relevant experience.
And there are times where a dogs health will be affected as it grows up.
For more on that:
Dogs with "happy tail" have the potential to actually break their tails by wagging them too hard and whacking something hard. This is extremely painful for them, and often results in having to have their tails docked as adults, which is much more painful than if it had been done when they were days old.
There is also the issue of the working dog, which is rare in today's society, but certainly still exists. Herding breeds like Australian Shepherds could get their tails stuck in a gate closing behind livestock, and hunting breeds could get their tails stuck in thick underbrush. Both of these situations would be terrible, and proponents of tail docking argue that working English Pointers and Setters (whose tails are not docked) suffer pain and infected tails because of this.
That is not to say that if a dog fits into this category it should automatically have its tail removed. A hunter who goes out a few times a year with the dog is far less likely to lead to a tail injury than one who is out practically everyday. As pointed in the original question the tail is an important part of the dog and it should only be removed if it is actually necessary.