Why does a male anseriform’s reproductive organ help with mating on water?

Male anseriforms (such as ducks, geese, and swans) possess an intromittent organ that is a vascularized sac everted from the wall of the cloaca. This organ is protruded by muscular action and retracted by an elastic ligament, which gives it a spiral form.1

This specialized structure facilitates copulation on the water, where mating can be challenging due to the aquatic environment. By allowing for internal fertilization, the organ ensures that sperm can be effectively transferred despite the instability of the water surface.12