Decoration. If you add (ugly) more tracks, you can really play roulette (see more detailed description).
You can build tracks underground, which accelerate a Minecart to a random speed. The Minecart then serves as a roulette ball and you bet on the corner of which you hope that the Minecart stops there. But I find it ugly, so I didn't do that. How this works can be seen in this video from sethbling: