I am now looking back at some of the games I played 15 years ago, and naturally noticing some obvious defects in my play. What seems common sense to me now – was completely unknown to me back then. I had simply not accumulated enough experience to get a sense of those many types of positions. Looking back on my endgame play, it's completely obvious that I was unaware of the subject of weak dark squares, space advantage, how to utilize a pawn majority, the importance of doubled pawns in the ending, etc. In other words I did not know what I did not know! I can only notice those defects in my understanding of chess now looking back through my games with the extra 15 years of experience. This reminds me of the quote by Rumsfield about unknown unknowns. To make things worse, 15 years ago, computers were not as readily available, so occasionally I would lose a game without even ever understanding what specific mistakes I had made.
It took me many years to accumulate that knowledge, so that those mistakes would become glaringly obvious. And this is exactly what a coach can do - use his decades of experience to point out weaknesses in the chess players understanding of the game. One can study the games of grandmasters and the try to of grasp their understanding of the game, but nothing can replace an experienced player looking at your games and immediately point out things you don't understand. That can literally save you years and decades of experience and help to avoid painful losses. Such a coach must be ruthless and as undiplomatic as it is possible. It may hurt your feelings now, can but that is the most useful thing a chess coach can do for you.