They are used to wrap the engine modules to give them a texture. Look at the last picture in @shiny_car photo post above. You can see them on the one engine in the rear (right side of photo). Hope this helps.
FWIW: I fooled around with a couple different placements of the different treads to see what I liked and varied from the original design. Feel free to experiment as @cavegod says and make it your own. :D