Mysterious Square

I have notices that most models have a squire on the rear bumper and it is driving my friend nuts on its purpose. Can anyone tell me what this thing is for?
Enlarge the photo and look at the righthand side or the bumper
http://www.familycar.com/RoadTests/MercedesS550/Images/Rear.jpg
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In the tool kit is a threaded tow hook. The frame has a threaded hole in it. That square is a cover that you remove and screw the tow hook into the hole. There is one on the front also.

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