ADVANTAGES OF WRITING ABOUT YOUR OWN FICTIONAL WORLD

1. You are in control of your own world.
2. Therefore you can set things up as best suits you.
3. You can populate your world with the characters you want to write about/for.
4. You can write in your own in-world jokes. 
   
5. You can use your own knowledge of geography, climate, history to create a good back-up story.
6. The more you write, the better you get.  A blog is a good way of advertising what you are doing.
7. You know the limits of your fictional world and can write to its strengths and, where apt, to its weaknesses as well.
8. You can set up its political world, which will feed into how your society is governed (and how your characters respond to that).
9. You can set up its criminal world/underworld, which will feed into your stories either directly (great fantasy crime stories, anyone?) or as information which affects your plot and which adds depth to it.  It shows there is more to your world than you are currently showing.
10. With everything set up, you can set your stories in contemporary times or go back in time to write a historical story, based on the history you have already invented.