Reason for one rear fog
The rear fog is always on the driver's side of the vehicle, and there should only be one. Two fogs could confuse it with brake lights. The fog light is on the driver's side, so faster traffic will know which side of the lane to pass on, and how much clearance they have on the left. Remember, in Germany people drive fast with or without the fog, so they need to know where traffic is at all times.
Mercedes also have the ability to keep a turn signal on, either the left or right, while the car is turned of. This is used for parking, when you park on a corner, or perhaps a blind intersection. So, during the evening, or fog, traffic can see where the end of your vehicle is, and avoid hitting you.