It really only takes a few times of doing these mirror drills with focus before they start to get it. From there, we work on mastering our mechanics with repetition.
In baseball, mechanics are everything. I often compare our mechanics as baseball players to that of a dancer's choreography. They must be refined day in and day out if we want to be able to perform at our peak. This goes for all aspects of the game from our fielding mechanics, to pitching mechanics, to hitting mechanics, and especially our mental mechanics. We never reach a peak of perfect mechanics; all we can ever do is continue to immerse ourselves in the process of Getting Better Everyday.
Mirror drills, specifically for pitchers, are drills that we can do in order to improve our pitching mechanics as well as improve our ability to visualize game situations.