It allows for the adjustment ofparallax so that the target is always perfectly in focus. I've never had a scope with parallax adjustment so I don't know how critical it is for hunting purposes. My feeling is it's probably not real critical for normal hunting distances and applications.

I did shoot my buddies rifle that had an adjustable objective. I wasn't that impressed, it's just one more knob to fiddle with when you probably have more important things to worry about when lining up a shot.