To address your question of ...what are they doing with them?:
There's people that just enjoy to create. Some are building a boat in their garage for 4 decades, others have their model railway hidden in the attic/basement where they spend weeks building a little pond, others again go big and design a whole world because they might like to philosophize in a more practical way & manner about large scale things such as population shifts and what-not.
While, as already mentioned, there's some people that publish their work and make money with it - there's plenty of work that will never touch the mind of anyone outside a small circle of friends.
For me it's a combination of both. I enjoy digging into theoretical problems and use the world I build as a framework for these questions while exploring them.
I also eventually would like to publish my world in the form of a novel or probably rather some interactive fiction - but that's way off for now :)