Drawing shapes and lines is an introduction to writing letters.
Talk about how letters are made up of shapes. "Look, that triangle almost looks like the letter A, all you need are the straight lines at the bottom! To make the letter O, you draw a circle."
Describe the direction and shape of your child's lines the next time they draw. "I see you're making a diagonal line. Can you draw a curvy line?"
Knowing the directions and shapes they're making will help your child learn how to write when they are in school.