This is a style of turnings called hollow forms, as the name implies they are hollowed out

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.