The first assignment that dared to enter into the third dimension.  Giving drawings, and animations, the illusion that they exist in 3d space helps sell to the viewer that it actually exists.  In drawing objects in perspective, an object in the background thats the same size as an object in the foreground will actually be smaller on the page.  Things look smaller the farther away they are.  In animating in perspective, the spacing from frame to frame follows the same principle.   

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A larger view of the clock
(click on the image abover to enlarge)