The adjustment you’re mentioning is most likely the knob for angle adjustment which are used if driving with a heavy load in the back. Some cars has suspension which levels the car automatically, for those the adjustment isn’t necessary.

Angle adjustment is useful even when not carrying a load. Angling the beams down onto the road at dawn and dusk provides good be-seen visibility without creating glare for other drivers.

Cars without manual angle adjustment have lost that courtesy gesture and just drive around glaring all the time.