I found this online. Maybe it will help you out a little bit.
The anal glands have no known function in the modern world. They are "vestigial" organs (like your appendix) meaning that they once had a function but as the dog and cat evolved, the anal glands lost that function. The best theory is that dogs once used the very pungent smelling anal gland secretion to mark their territories in the wild. Wolves and other wild cousins of the dog are known to do this. Also, dogs will occasionally express (meaning to squeeze secretion out) their anal glands when they are frightened, so it is thought the secretion may have been a defensive repellent (like a skunk). |