It's trying to ease the problems surrounding contextual overrides.

Having color="red" and other inline styles is a bad design pattern, but AM only superficially resembles it in that it uses attributes. Whether you give an element the attribute of class="nav" or just am-nav, it's accomplishing the same thing: defining that it's a "nav" element for purposes of styling.

Not to be insulting but… are you unable to read text? Because the problem (which has nothing to do with visual clutter and has to do with modularity and contextual overrides) is explained from the third section "Contextual overrides".