Adding track accessories will increase the immersiveness of the gameplay experience. It can also make the game just plain more interesting.
About this creation
Mithryl and I discovered early on that a track on its own is very boring. The more details and accessories you add to your track, the more fun you will have with your games. Here are some common track accessories:
Anti-Vehicle mines like this one are dropped by opportunistic players to try and foul up their competition. They form permanent obstacles on the track when deployed.
Barriers such as these help to outline the boundaries of a track.
Obstacles such as these help make the game more interesting, as players have to be on the lookout for more than just eachother when racing.
Finish/Start lines like this clearly indicate where the course ends and begins. They don't have to be this elaborate, but it doesn't hurt.