Every player is unique. Each batter has a different swing. Each pitcher has a different delivery. As players get older, hopefully they are being helped to learn what makes them the best they can be.
Hitters are learning what the best place is for their hands to be when the pitcher is about to release the ball. When do they need to get their front foot on the ground to insure they will be in the best position possible when the pitch gets into the contact area. The biggest issue you see with hitters, especially youth, is their lack of being taught what it means to be “on time” when the pitch gets to them.
Pitchers need to know what makes them tick too. What they need to feel pitch after pitch. Where they need to feel smooth. Where does the separation of their hands need to happen. What is their best arm stroke in the back.
All this comes with rep after rep. Learning what works best for you/them. Hopefully at some point in a players life, they end up in front of someone who understands this and can help the player to understand themselves and what makes them the best possible player they can be.
By knowing yourself and your swing or delivery will allow you to make quicker in game adjustments. If you take a bad swing, you need to be able to make an adjustment on the next one. If you throw a pitch up and arm side, you need to know yourself so well that you can fix it on the next pitch.
The ability to make quick adjustments is a huge part of this game. It is in fact one of the biggest things we work on at the pro level.