Why do dogs have a tail?

One reason is to cue others to their "moods". Dogs pay attention to tail placement/movement to determine friendliness/aggressiveness/etc. of pack members. Some domesticated dogs use their tails to aid humans-pointing while hunting for example. And they are just darn CUTE when wagging!!! Im sure someone will come along with a much more scientific reason shortly. :) A dog's tail is an extension of the spine. It helps them to keep balance when they run.