Soup, maybe. But they're mostly fat.

Turkey tails are quite tasty...a lot more meat than chicken tails, and the cartilage and fat makes them quite tasty and also means they are great for making stock. Many people buy them this time of year (along with extra necks or wings) to make stock to use for gravy.

Thanks, rachelellen; I had thought they were almost all fat & therefore not good for much.

This is a part of the bird that certain people did not want to eat because of the looks of it (think plantation). Other people (who were hungry) became real cooks because they were forced to make something for their family to eat and enjoy.
Instead of asking why someone would want to buy/use/eat them, you should ask the butcher/grocery store why they sell them. If you talk to the source, you'll get your questions answered.