The more details I put into my fictional world, the more real it is (though I'm increasingly glad I don't live in the Fairy Kingdom as I've written it. Far too much trigger happy magic is going on for it to be of any benefit to anyone's health!). The more real my fictional world is, well I find it backs up and deepens my portrayal of my characters. The world seems real, so do they and I think my stories read better for that.
I also like adding my Fairy Kingdom stories to the overall fairytale genre and see it as a way of giving back as a thank you for all the tales I've loved over the years. The best way to support a genre is to keep it going!