When you first start learning how to play tennis, you're so focused on just trying to hit the ball. It's hard to concentrate on co-ordinating better footwork and positioning.
The goal is to approach the ball the same way each time and hit at a consistent contact point. This is inherently difficult as long as the brain can use the wrist to make the adjustments for you.
The solution to this is blocking the wrist. The brain automatically focuses on positioning the body first which trains the brain to use the feet to make adjustments and for power. With the wrist blocked, your practice time becomes dedicated to positioning and finding the optimal contact point every time you hit the ball. Once that technique and response is ingrained into mental memory, you will never think about how to hit the ball again and naturally position yourself better.
This video provides a lot of helpful tennis training tips and explains this in detail: