Babies develop their movements, as a reflex. These primary reflexes are necessary for survival like the suction reflex. A baby can suck from birth to get food, you don’t have time to learn this first. Babies also have reflexes to help them learn to roll, crawl, sit and stand.
By moving a lot, the movements are controlled from the brain and they no longer go by themselves. The primary reflexes disappear and make way for controlled movement. For this you have to make a lot of movements. By moving a lot, their movements will be controlled by their brains.
When there the child does not have the ability to move around in this way or there is a trauma, such as illness in infancy, it may be that reflexes remain active The body will move without control. These reflexes are stronger than a controlled movement, because survival is of course important. You try to suppress the reflexes and compensate, but a good foundation is not being made and this will cause problems.