Most vehicles will be born and die bone stock. The internet greatly over-estimates the number of vehicles that get aftermarket lights (LED dome light replacement bulbs from Autozone notwithstanding).
You are supposed to adjust them based on the weight distribution, but people tend to use the highest setting, because that's the best for them. But if they have a lot of weight on rear, the beams are too high and can dazzle other drivers.
And sometimes the optics are simply misaligned because of shock, wear, vibration, ... even dirt can change the beam shape and be problematic. This is especially true for HID lamps, which are closer to a point light source and require higher precision optics compared to halogen light bulbs. And because it mostly affect others, these problems tend to go unnoticed by the driver.
