Graphics are, after all, what the user sees. The real power of a GUI flows from the fact that objects on the screen behave like objects in the real world. You can move a window on the screen, just as you can move a piece of paper on your desk. Clicking in a button on the screen makes something happen, just like pushing a button in an elevator. Things on the screen only move if you move them, just like things in the real world.
Most users consider GUIs to be simple and natural.
They're half right. A GUI is natural, in that it embodies properties of the real world. However, those properties are not particularly simple. The reason that people find GUIs easy to use is that the human brain has evolved to function in the real world. By mimicking the real world in certain ways, a GUI can leverage the power of the human perceptual system, thus making the functionality of the application easily accessible to the user.