Tennis is a game that requires tons of repetition. Acquiring a motor skill requires practice, practice and more practice. Allow me to get nerdy for a minute. What happens is the brain sends electrical impulses through the nerves to your muscles. There is no such thing as muscle memory, your muscles are dumb and only react when they receive an impulse from the brain.
When first learning a new skill the electrical impulse is weak when it reaches the involved muscles causing the motor pattern to be uncoordinated. However, practicing the same thing over and over again causes the brain to send the same impulse repeatedly. When a particular electrical signal gets sent enough times the body senses that this signal must be something important and it wraps the pathway of the impulse with an insulation called myelin.
Myelin is basically like insulation and it allows the signal to traveling from the brain to the muscles to do so faster and remain strong. The more a person performs a skill the more myelin is wrapped and the more automatic the skill becomes. The is especially true from young children who are in the optimal window of opportunity to learn new motor skills due to adaptability of the brain and nervous system. We all have motor patterns that are myelinated. Think about walking for example, when you were a toddler the walking impulse was sent so many times the patten became coordinated and automatic. Learning the motor skill of how to hit a tennis ball is no different.
Now watch the video below and you will understand exactly why we try at times to not have the kids in tennis camp always just stand in line waiting. Certainly there is a time to use waiting in line as a chance to recover from a difficult drill. However, when the drill is designed for skill development shadow stroking provides way more repetitions which means more impulses being delivered. I tell kids all the time they do not even need a tennis ball to practice. They can go home and shadow stroke in the driveway or backyard until their heart is content.