When you or someone else is driven to physically hurt a child in order to discipline them, it hurts the child both mentally and physically. The child now thinks that it is okay to be physical if they are acting out, and they consequently have a lower self-esteem and are more likely to do destructive things the older they become.
A child is at the mercy of his parents or guardians. If the caretaker chooses to use corporal punishment it will get the child to behave in the manner expected of them. Although the child may physically behave in an appropriate manner, the child is going to have a lot of internal emotions blowing up without any form of release. Since the child is unable to act out in any type of physical manner, they are likely to act out in a way they can control. If they do no nothing, it leads to depression. If they act out, the one thing they can control is whether or not they are going to continue living this life.