That is a great question.

The goal of charting your pitches is to help you find out what is working and what isn't.  It helps you make sure you are not predictable.  It lets you know if you are falling into a pattern that the other team can pick up.  It helps you track 1st pitch strikes and what worked or didn't to each hitter.  It is not a cookie cutter approach, it is personalized to what pitches you throw and helps you identify what you need to work on in between starts or outings in the bullpen. 

Charting the hitters helps identify their strengths and weaknesses.  Whether they are aggressive or take a lot of pitches. How they handle in fastball, curve etc.  Then you can create a plan to pitch against each hitter.  It might change depending whether it is the first time through the order or 2nd, 3rd etc.

You are exactly right that mixing up your pitches, changing location of your pitches and being aggressive in the strike zone is the key to being a great pitcher.  

The more pitches that you can throw for strikes and throw them where you want to will be a huge factor in your success.

Watching the MLB pitchers pitch is just a learning tool to see how big league pitcher's think and how they put their pitching plan together and execute their plan in the game.  The Playoffs is usually the best baseball that is played all year long. 

Having a plan is not a defined pitch sequence as much as knowing what the hitter's weaknesses are and how to exploit them. Having a plan helps build confidence and keeps you focused when you are on the mound.