I can see this is a stop of some kind but what is the usefulness when the open/close mechanism seems to dictate how far the headlight will open.

And, is there a special way to adjust this other than just a stop in the open position?

(and thanks to the guy I lifted this picture from..I keep forgetting to take one of my own)

It is adjusted with the lamp in its highest position by motor/link position.
It keeps the lamp from bouncing.

^ I agree.