Mud flap positioning

[getty150]

Very puzzled...
Why are X3 (BMW) the mud flaps positioned in FRONT of the tire and not to the rear of the tire, back when I had my old Buick.

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[getty150]

The Air Dams (not mud flaps) are there to deflect/protect/redirect the circulating air, road particles at the bottom of the flat surface of the under car and tires and engine, wheel well and even breaks.
These dams are engineered for fuel economy, aerodynamic and reduces noises...

And I was beginning to dis-count the need for the "dams".

However, as I see on some truck models this deflector (dam) is not just "protecting" just the tires or wheels as for the e83 but runs from front passenger wheel to front driver wheel at the bottom of front bumper.