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
If Facebook used SASS they may still appear to have the Facebook blue defined 261 times and have 548 unique hex values. The redundancy is only removed in the preprocessed files, when the files are processed the values are transformed and inserted back into the document
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