They are usually removed to stop them catching and bleeding, also they often grow very long and round digging into the dog's leg as they don't come into contact with the ground so are not worn down naturally. Some breeds mention in their standard that they should be removed or left on. Some breeds actually have to have rear dew claws, and some have double ones on the back. I had a Border Collie who had double ones, but one on each back foot had no nail on it, it was just a sort of gristly growth. He had them removed when he was castrated.
i saw a Dobe x Rott today who had back ones removed about six weeks ago, and it has healed very well. He is an adult rescue dog, and they were very loose and hanging, so in grave danger of being caught.