Watch the TV program called "Super Nanny" and you will see why.
First thing you will notice when you see the mess she walks in on, is how almost of the kids physically lash out with their hands and feet, and also verbally.
Children test the limits at a very early age. It establishes exactly how far they can go, and what limits they can push for dominance. Until it is established that either the children are in control of the parents or the parents are in control of their children.
Whatever a child establishes (self control or no control), it has become part of a permanent personality trait by age 10.