Hello Crash,
I have a 3-12 Burris with target knobs and parallex adjustments. I shoot it from 200 meters to 500 meters. The parallex helps focus the glass, so to speak, and it also is critical with the position of your eye in relationship to the center of the eye piece.
Example: Set the parallex for 300 yards and look through the scope at a target 300 yards away. Now, move your head just slightly from one side to the other so as not to be centered with the scope. The crosshairs will remain more or less on target. However, now set the parallex to 50 yards and do the same test. First of all the target will be a little more blurry and when you move your head slightly, the cross hairs will be off target. I have 6 Burris handgun scopes and all but one have Parallex adjustments. I have 4 fixed 10 power rifle scopes without parallex. I really wish I had the parallex on those scopes. The scopes are great quality and the price was extremely right when i bought them.
Harold Clark