Style guides are inherited from the old days of print. Defining typography and inks to ensure the next magazine looked like the last one. They were on paper, for paper.
Designers were the ones who owned these documents, and they've inherited them for the web. Sure, sometimes style guides are put up online, as their own website. But they're still not in the code.
This means that designs get set, then chucked over the fence to the folks who actually implement them, often leading to painful rewrites when design changes happen [Blue. No, yellow!].
By making Sass act as your style guide, breaking design into variables and mixins, a site becomes easier to maintain, and often options get opened up that someone on the visual side wouldn't have thought possible.