Since most of the domestic dishwashers are used not only for dish washing but also storing bowels and dishes after washing. So dishwasher should be free from sewage backing up. If the drain line of dishwasher is connected to kitchen drain line directly, sewage backs up from kitchen drain will go to the dishwasher directly and corrode all dishes & bowels in the dishwasher. The left side drawing is showing what is the air gap and the basic diagram of dishwasher drain line flowing.