???? ?? ???? ???? ????? ?? ??? ?????? ??????? ?? ???? ????????

What's the rationale behind styling flat/straight boxes to cast curved shadows on flat/straight surfaces?

^{Image credit: http://cjwainwright.co.uk/webdev/liftedcorners/}

Sure it makes the object stand out, but this shadow is physically impossible. The paper is straight (no curled up edges), and the webpage has a flat background.