Why is it important for designers to use the human figure in
evaluating scale within a design?

By using scale to make an element larger than others appearing with it, you can emphasise that element. Not only can you make an element stand out this way—you can also use scale to create a sense of depth (since nearer objects appear larger to the human eye).

Hope this Helps

photography had emerged as an elite art pursued by a few creative individuals for a few wealthy patrons. but now, said bauldelaire, it was succumbing to "the great industrial madness of our times." it was becoming a mass production industry that catered to middle-class tastes, including a taste for gadgetry. the stereoscope, the first 3d imaging system, had turned looking at photographs into parlor entertainment. a "thousand hungry eyes were bending over the peepholes of the stereoscope as though they were the attic-windows of the infinite.