Jason Santa Maria, writing for Typekit:
Why do we even need displays weights? Most typefaces that were designed for paragraph sizes work best at those sizes. But when you size these fonts up to use as a large headline, all of the design considerations that make it such a good text face become less appealing. Odd contrast in the strokes, larger spaces between letters, and more, can make a font readable and pleasant in running copy, but prove distracting when enlarged.