Dogs are animals, whether we want to believe it or not, they are descendants of the wolf and still maintains some natural instincts which are coded in their genetics. One of those natural instincts is to own territory. Yes, animals do invest in real estate, they generally mark the area which they are dominant in with their urine. So if you have a large back yard and you see your dog urinating on that new tomato tree you just planted it is because he may see that plant as a foreign object that is new and is within his domain. He will therefore leave his scent on that plant to let it know whose urine is top urine around.
Dogs will also mark over other animals
Let us say you have a a couple dogs and a cat. If these animals are outside roamers you will notice a pattern in what takes place when they urine. If a female dog urines and a male dog spots her urine, he may venture to the spot where she had urine, give it a few sniffs and then leaves his mark over hers. The male dog would have done the same if the cat had also passed urine. This action is just another way to establish dominance, sort of like your dog saying "Only my urine should be smelled around here."