I don't know that they have a practical use but they are fun to make. This one is Walnut.
This one was fairly plain looking so I added ridges all the way around it. I'm not sure what kind of wood it is. My wife bought me a big box of random sized and species bowl blanks.
This hollow form is made from a wood called "Purple Heart". It comes from Central and South America. If left untreated the wood turns from this purple color to a brownish color.
It's a little hard to see in this picture but this is a bark topped hollow form from a Silver Maple tree.