In my opinion, this is one of the most important features of CSS preprocessors. Wether you're doing rapid prototyping and working on the real deal, having a set of variables for colors, fonts and whatnot helps you a lot controlling the look and feel of your site and avoid mistakes that even Facebook did [1]

This is one of the main reasons why foundations like Twitter Bootstrap or Compass are awesome but unfortunately people are so lazy they don't even change the color variables in Bootrstrap, hence the "yet another Twitter Bootstrap" site.

[1] Facebook had 706 CSS files, they declared the Facebook blue color 261 times, used 548 unique hex values for colors, had 6498 declarations of color. http://www.lukew.com/ff/entry.asp?1379