Why do they cut off dog's tails?

This weekend, while walking our dog Biscuit, a black spaniel came up to greet her. It was cute to see the spaniel wagging her tai, but it had been docked (removed) so close to her rear that she was basically wagging her butt. I had wondered before why people dock the tails of some dogs, and the answer I always heard was "Just because." There was no logical reason to be had, no matter how many times I asked. Finally, to day I think I have found the answer and it makes some historical sense.

A number of working gundog breeds have to hunt game through heavy vegetation and thick brambles, where their fast tail action can easily lead to torn and bleeding tails which are painful and extremely difficult to treat. Docking the end of the tail eliminates the risk of injury.

Working terriers are docked for the same reason. In addition, terriers which are bred to hunt below ground for purposes such as fox control, have their tails docked to a length which is more practical when working in a confined space.

Other non-working breeds which have an enthusiastic tail action, are also liable to damage their tails, even in the home.

Since docking was banned in Sweden in 1989, there has been a massive increase in tail injuries amongst previously docked breeds.

I am still not sure it makes complete sense, but at least there is a real reason now.