I believe the answer your looking for is that sight height is important in calculating trajectory because you are using the line of sight as your aiming reference. If you disregarded the sight entirely and instead relied upon your actual cartridge performance to aim same way as an old artillery man who could detrmine impact by the distance, height and angle of the cannon we would not need to take sight height into effect. However then you would be required to take into account height of the barrel from ground, altitude, and angle of barrel relative to target, all very difficult things to do without tons of practice and all of which would require spotters and many rounds to determine the actual performance.However since its much easier and FASTERfor us to aim by eye with ascope and then fine tunethe point of impact, sight height is important. Its your line of sight which provides the referrence to calculate trajectory and adjust the barrel and bullet trajectory with the aimed for target.Its only because it seems so intutitve that your friend feels he doesn't need to know sight height. Same reason why its much easier to catch fly balls in the outfield when you hold your glove directly in your line of sight( in front of your face)andwaist high basket catches are considered risky show off maneuvers. Think of catching a ball as the inverse of hitting a target with a rifle. you always gotta know where your gloved hand is relative to your face( and why you are always told to get in front of the ball so its path is directly into your line of sight and thus lowering the sight height and minimizing any calculations your brain would have to do and thus making you react faster.) Imagine seeing a ball hit and the simply running to a spot where you calculated it would land and sticking out your glove without looking and expecting the ball to land. like an old fashioned cannon man it would take a lot of balls until you finally put yourself into the right spot for that to work.As with a scoped gun its much easier and faster to simply get into the general area where the ball was hitand then fine tune once your there so the ball is proceeding directly into your line of sight and put your mitt in front of your face and pray. Hope it helps you visualize the difference. Help me if I'm wrong guys, just trying to put it ionto a way that hecan feel and understand the difference. take care.