In my case, I've been able to envisage how local communities work, devise a Highway Code for the magical realm (huge fun to write!) and I hope at some point to produce other works. I can envisage a guide to magical law, Palace etiquette and that kind of thing.
I am thinking of developing these kind of works into future books (possibly ebooks) and see how things go but I do know now it is satisfying to invent things like this that can back up my Brenebourne novels and other stories.
I love The Discworld Companion for its details and I am pleased I can genuinely invent things for my own creations. I hope it is a sign of a very active imagination I can use to promote and back up my main works. Certainly it is good to have the option!