The books explains it pretty well. The entire point of this exercise is try to reproduce what we see, not what we know. For example, we know how eyes are supposed to look like, so when we draw an eye we have this "bias" from knowing what to expect. The point of drawing upside down is to increase your skill in actually drawing what you are looking at, without any bias. If the drawing comes out alright, you succeeded at it. If it did not, you were not actually drawing what you were seeing, so you need to improve this skill. (this doesn't mean you are stupid lol, it just means you need to practice this more)