When it works out well! That is, I read back what I've written and it all makes sense. The images I had in my head about my fictional world are conveyed well enough on paper and would make sense for any outsider coming to it for the first time. I like getting a deeper understanding of how my worlds and my characters work, think, believe. I think it makes writing them more realistic, much more likely to have that ring of truth about it, which I think is a massive asset for any story. Even the silliest funny story has as its point, its ring of truth as it were, the wish to entertain. Does it do so?
I ask myself questions about how different aspects of my world work from how the fairy government works (to how come there was a fairy government in the first place and not say a witch or wizard based one). I enjoyed fleshing out how the villages are made up and how decent planning disputes can lead to the fatal use of magic (and usually the obliteration of the local mayor, who is generally held to blame for not sorting these out properly in the first place). I am very glad I don't actually live in my Fairy Kingdom (I can see where Eileen comes from in her rebellion against it but then as her creator I ought to! I'd very much share Eileen's attitude here. I don't share it on everything. She is far too awkward for her own good but that is what I love about her from a writing viewpoint. That kind of character is fun, drops themselves in it, has clashes all over the place. Loads of material to use there!).