So their footprints can be distinguished from a crows!

Because they swim?

It propels them through the water better. Ducks and geese don't "swim", they float. Their webbed feet motivate them through the water. They also tuck their feet up into their belly down on migration flights to keep them from freezing.

They use them as fins to help them swim quickly. Just like why boogie boarders use fins.

So they can quickly motor away from people and their pet crocidiles!

1) "Why do ducks have webbed feet? To stamp out fires. Why do elephants have flat feet? To stamp out burning ducks." Source and further information: http://www.funny.co.uk/jokes/art_8-1441-Why-do-Ducks-have-Webbed-Feet.html 2) Evolution. They developed webbed feed as an adaptation to their living mostly on water: "Gansus yumenensis takes its name from the Gansu region, where it was found, and the nearby city of Yumen. According to Dodson, Gansus is something of a lost species, originally described from a fossil leg found in 1983, but since largely ignored by science. The five specimens described by Dodson and his colleagues had many of the anatomical traits of modern birds, including feathers, bone structure and webbed feet, although every specimen lacked a skull. "It appears that the early ancestors of modern birds lived lifestyles that today we would stereotype as being duck-like, heron-like, stork-like, loon-like, etc.," said Jerald Harris, director of paleontology at Dixie Sate College of Utah. "Gansus likely behaved much like its modern relatives, probably eating fish, insects and the occasional plan. We won't have a definitive dietary answer until we find a skull." The skeletons, headless as they are, offer plenty of evidence for a life on the water. Its upper body structure offers evidence that Gansus could take flight from the water, like a modern duck, and the webbed feet and bony knees are clear signs that Gansus swam. "Webbed feet is an adaptation that has evolved repeatedly in widely separate groups of animals, such as sea turtles, whales and manatees, and would only hinder climbing or landing in trees," Harris said. "The big bony crest that sticks off the knee-end of their lower leg bones are similar to structures seen in loons and grebes. These crests anchor powerful muscles needed for diving under water and swimming."" Source and further information: http://www.upenn.edu/pennnews/article.php?id=968