the path of the wind dumps stuff on the rear window. And or the SALT mist we get here in NYC...or the rear kicks up salt mist that does dry and freeze up on the rear windows.

It's nice to see the REAR and have it clean when you want to watch for a LEO or just to check your rearside in inclement weather and see if the driver behind you is giving you proper space..

I dunno but I can think of 10 zillion ways why I would rather have a rear wiper than not.......