So it can have a comparison for the tilt of the the car, rather than the specific height. If you just loaded up with four fat bastards then the car would lower but still be level. Conversely, if you put three bags of cement in the boot with just a driver, the car would dip at the rear and the lights would dazzle. It needs to know the relative compression of the suspension front-to-rear in order to vertically aim the headlights.
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