The brain develops fluency with practice. At first, learning is conscious and laborious, gradually becoming automatic. When the mechanics of writing or reading become unconscious, the cortex allocates more attention to the meaning of what is being written and read. Children need to connect 40 speech sounds with their letters to learn the alphabet code. The more practice they have, the more automatic those associations become.
Motor memory is a separate, very enduring brain system.
Once we learn to ride a bike, we don't forget. Simple keystrokes are linked to sounds and letters. As the fingers begin to learn their tasks automatically, children get into the fun of writing. The brain processes wholes AND parts simultaneously. In other words, writing uses more of the brain.