Good Coding Practices Help

In the good coding practices tutorial there is a section on Dependency Injection. I really didn't understand how interfaces would lessen the dependency of playerinput to playerphysics. It seemed like a lot of extra code where you could just use 2 classes. This is what I interpereted her instructions to be.

(in Unityscript)
Class PlayerInput{

funtion applyPhysics(IActorPhysics actor){


}

}
Interface IActorPhysics {
function move();

}
Class PlayerPhysics implements IActorPhysics{

move


}



Here is a link to the tutorial:
http://unity3d.com/learn/tutorials/modules/intermediate/scripting/coding-practices