The elevation knob has two parts to it, and that screw holds them together in alignment. With that screw tightened, the whole knob will still turn and lock into place on the different settings. That setup is made so that you can adjust the sights for known distances. For example, you are shooting at 300 yds and then want to shoot at a 600 yard target. Instead of holding high, you would turn the knob from the 3/6 setting clockwise until it came back to the 3/6... The rear sight will now be raised for shooting at the 600 yard target. Then if you go back to 300 yards, you turn the knob counter clockwise all the way until you get back to the 3/6 setting. (Provided you zeroed correctly for this type of sight system)