Charting is a way of tracking different types of information for hitters and pitchers. Charting can be done to analyze a singe at bat, a game, a series of games, or a season. You can think of it as an extension of the score book. When charting your not concerned with statistics that you normally calculate from your score book. What you're interested in is information that you can use to help you coach your players.
I know many coaches and parents who can tell you the batting averages, homeruns, ERA, etc. for everyone on the team. Many coaches spend a lot of their time calculating those stats. What do those stats really tell you? Mostly they tell you what you already know. You know who is hitting the ball well and who is not. You don't need batting average to tell you that. Ask many of those same coaches and parents what the strengths and weaknesses of an individual hitter or pitcher are and you'll find they often have no idea. The focus on stats often takes away from where the focus should be, building skill. Charting allows a coach to analyze performance after the game of individual players and from that come up with a practice plan that attempts to help the player build the necessary skill in areas of weakness.
Charting your games or a team scrimmage will give you some valuable information that you can use to work with your players on the mental part of playing baseball. It will help you help them in developing an approach to the game.
When you chart a pitcher, you have the information to go over the game hitter by hitter, pitch by pitch. This will allow to to discuss the approach to each hitter, pitch selection, pitch location and pitch counts. Rather than telling a pitcher that they'll get in trouble more often when they get behind in the count, you'll be able to show them. See pitching strategy for more information.
With a hitter you'll be able to go over the pitch selection during each at bat, approach to each at bat, and situational hitting. See hitting approach for more information.
Charts can be used by players during a game. In between innings a pitcher can go over previous at-bats of the hitters that will be coming to the plate the next inning. This will provide him more information to determine how he will approach those hitters. A hitter can also use the charts to not only determine what a pitcher has done against him up to that point in the game, but how he has pitched to all the hitters.
If you face the same teams multiple times, charts from the previous games can help you prepare for both the pitchers and hitters that your team will face.