It's actually called the 'grease gland', and they use that secretion to mark territory. You'll notice sometimes that they're rumblestrut (make a purring sound while waggling their bottom) and will drag their bottom on the ground.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry