A critical element of any servo system is the feedback device - after all, that's what makes it a servo to begin with! How about a very simple example to start off with: I have a bow and arrow, a target 30 feet away, and I left my glasses at home. So while I do see a large round "thing" in the distance, I have trouble making out the edges of the rings on the target. My feedback is not very accurate at the moment - so I'm likely not going to hit the bull's-eye. I discover my glasses in my pocket, slip them on - and now I can see the target much better, and I at least have a better chance now of hitting the target. Yes, there are other factors, environmental, arrow construction, etc., but you get the point (pun intended)!