Well, I would think it would be to get all the germs off of the dog so that there is a lessr chance that someone in the house will get sick. Also, the rain could get the dog sick, and if you let the dog back in without cleaning it, someone in the house could get sick. I assume that you probably care about your dog, so this is most likely done for the care and safety of your dog. There is absolutely nothing wrong with doing what you're doing. It is probably just done for everyone's safety.
Because it displays the number of organisms at each tropic level.
