Well the list can go and on and on. In short they are full functioning Linux computers. They can be used for mini servers and media players plug and play if you buy everything ready to go.
Hobbyists uses, there are hundreds of uses. I have several on one project or the other. I use two for different rooms with Kodi on them tied to my media server. Kids and wife and watch any movie I have on the server at anytime.
They're in an awkward position between the Arduino family of products and netbooks while not really covering the breadth of either. Extension/expansion of capabilities tends to be relatively spendy and most who like them seem to end up with several of them for different functions where a more comprehensive solution might be better.