a scrollbar when done right is the most natural form of moving down a page.

most scrollwheels have "notches", so if you move the wheel one "notch", its rarely the exact amount you wanted to move. the middle click is also not ideal, because it starts moving the page based on the current mouse position relative to the original position. so if you accidentally move too far away from the origin, the page shoots up or down.

with a scrollbar done right, you have fine grained, manual control of the movement of the page.