The top half of the reflector is the standard low beam pattern to prevent blinding oncoming traffic.
The standard H4 incandescent bulb has a shield on the low beam filament for that reason.
The position of the light source is also important because the reflector has a focal point. The High beam filament is in the best point focus. This reflects the light furthest down the road.
Most vehicles switch between low and high beam filaments because of power consumption.
With LEDs its possible to leave the low beam LED on whilst the high bean LED is on. This gives both light patterns at once and more light on the road. This can be done internally in the LED lamp.
Sent from my SM-G930F using Tapatalk