All of the above mentioned ideas are possible reasons for the piece. My State only requires body work over the top of the tread, not down the back. One of the screws that hold it on, also attaches the fender liner to the frame. There may be a specific dirt-wind trail at that spot, the piece redirects flow to stop it. I have sent a picture with the fender off. I took the picture when I was installing the trailer hitch.