Anyway, cloacas aren't especially common in mammals, but for the other amniotes and the amphibians, that's just how they live. The word derives from the Latin for sewer or drain, which has roots in "to cleanse", and it may also sometimes be referred to as a vent.
Some animals have a phallus which emerges from the cloaca, which helps out with reproduction. It's inserted into the female's cloaca.