The side wheel focus is more of a range finding device. It works pretty much the same as a scope with an adjustable objective bell, but the adjustments are made with the wheel not the bell-end of the scope. All scopes exhibit what is called parallax error at some range. This occurs when the image of the target and the image of the reticle are in two different focal planes. This can be seen easily by just moving your head around while looking into the scope. If there is error, the reticle will move around in relation to the target. If the parallax setting is correct, it won't move. What the side wheel does is provide a means for adjusting the parallax setting, and in doing so determine fairly accurately the distance to the target. Adjust the wheel until the reticle and target picture are in the same plane of focus, and read the range. Theres usually markings on the wheel that indicate range to target. Once the range is determined, the scope can be adjusted properly for that range. Normally you'd need to set up the scope beforehand by setting a zero then walking the scope through different range increments and creating a range card indicating how many clicks up or down from the zero are needed to get the reticle centered on the target.
