CSS, although a very cool and powerful language, leaves a lot to be desired in one key area: abstraction. Not coincidentally, this is the area that Sass is best at. Chris Eppstein has an excellent blog post (<a href="http://chriseppstein.github.com/blog/2009/09/20/why-stylesheet-abstraction-matters/" rel="nofollow">http://chriseppstein.github.com/blog/2009/09/20/why-styleshe...</a>) explaining why abstraction in stylesheets matters.