Good question! I noticed that strap right off when my '02 Silver was new. I believe I have read an explanation somewhere but can't recall it at the moment. I thought it might be useful for holding a fire extinguisher or a big flashlight, but I couldn't get it to do either. My back finally gave out from leaning into the trunk trying to figure the damn thing out.
It was posted, on one of the other Prius sites, that the strap and the small well in the trunk floor by it, are for holding an emergency kit. Supposedly, all Japanese cars, and maybe some European cars, are required to have one in the car.