[MIKarlsen]

Showing someone (or another dog) that you're excited or scared ("tail between his legs") is a very potent social mechanism I think. I don't think it adds much to the motor skills of the dog, besides maybe giving it a tool to keep flies and insects away from the ol' pooper.

[NeedMoreTea]

Watch something like a greyhound or German Shepherd at full throttle and you might reassess your view about whether it adds to motor skills. It seems to be a very active part of a full speed sprint.

It's also a very subtle tool for social interaction as you note - the difference between tail wagging whilst up, horizontal and down, or wagging just the tip and so on. There's quite a lot of complexity hiding there.

[toasterlovin]

Anatomical features often serve multiple purposes. Scent glands in many animals are evolved out of pre-existing features. We talk and chew with our mouths. Etc.