Could be there for aerodynamics rather than downforce like is needed on a race car. I have seen video of some wind tunnel testing on other cars (not the HAH) where they introduce a thin like of smoke into the wind path to see how the air flows smoothly over the shape of the vehicle. As I recall, there is ususlly some turbulance or "eddys" created just after the stream exites the surface of the rear of the vehicle, which I assume could be translated into "drag". Maybe the spoiler changes the direction of the air stream upward slghtly to make for a cleaner exit, less turbulence and therefore less drag. Just my theory, don't know if there is any truth in this. I do know that the Honda engineers worked there magic to get the drag coefficient down to .29, which included the design of the wheel spokes.