All humans struggle for power and control over their own lives and young children are no different! Ideally we want our kids to do what we need them to without us having to do anything more than merely ask. Let's not kid ourselves, that is just not going to happen most of the time. "I said so" or "you have to" is about us asserting our power over them and can feel disrespectful to the child.
Our children have little to no control over much of their daily lives. That is why most power struggles revolve around their physical self or body.