This behavior is natural in dogs. They cover up their "waste" so that an enemy does not find them, from the scent of the poop or pee. It is an instinct handed down from wolves, who hid their scent by covering up their feces in leaves, sticks, soil, or other nearby natural materials. They also rolled in animal carcasses to hide the scent of them. Your dog is just using her natural instinct to protect herself from predators.
Another things that dogs do, called marking, is when the dog pees in a certain area. They do not cover it up, as it is marking a territory as their own. Male dogs usually do this on trees, wooden posts, furniture, and other things. Female dogs do this in heat. Dogs do not poop to mark something, only pee. When you dog pees or poops and then covers it up, this does not mean she is marking her territory. Just wanted to add it so you don't get confused. :)