Why do most cars have cross protrusions in the middle of their doors?

This is what we usually call "waist line", and its main function is to prevent collision. The waistline is the line on the doors on both sides. If the waist line is close to the window, it is called window line. Or some cars have wires on both sides of the doors, and there are wires near the windows, so the windows near the windows are called window lines. In fact, it is almost a line, extending from the front of the car to the rear of the car. Different positions have different names.