They defy our notions of what is real and demand to be understood. We are told that the diagonal lines are parallel, but we have nagging doubts.
Illusions are really indicators of our ignorance about the workings of perception.
They deceive us, yet teach us something about the process of perception. The Necker Cube appears to change its perspective willy-nilly without regard to an external cause. Since the image does not change, the cause must reside within the observer. This indicates a perception process more complex than first imagined, perhaps in league with an underlying neural mechanism. Since the first model of human vision, scientists have tried to fathom the complexities of the perceptual process.