Parents use speech to control a child’s behavior.
The child, before the advent of language is prone to be an individual who responds directly to stimuli.
When he is hungry he wants to eat now. If he is angry, he will cry and strike out. If he wants something, he will take it (if he can), etc.
During this time, speech like other cognitive and motor skills is steadily developing. We are talking here between two and seven years of age.
Quickly, parents are using speech to control the child’s behavior. “Come here, Edward,” they say, or “Put your shoes on.”
Whether this acts as a model, or whether it’s a genetic trait, the child begins to control his/her own behavior using speech.
Like all motor skills that are newly learned, it has to be physically acted out. Hence the child speaks out loud.