|Dog's urine is used as a scent marker for other dogs. Many male dogs and some female dogs will urinate in multiple places to establish their scent on their territory and say "I was here."
A male dog lifts his leg so he can urinate on vertical surfaces (the proverbial fire hydrant). Why urinate on a vertical surface? Bruce Fogle, DVM, in his book Know Your Dog, suggests the scent of urine on a vertical surface will generally last longer than the scent on a horizontal surface. The scent is also more at nose level, so the next dog coming by will be less likely to miss the scent.