I understand that it eases building CSS frameworks. But, really on the non-framework front end developer side it really has a flawed concept.
CSS was made to separate the style from the content and layout. And we have the effective strategy of creating unique class names per distinct and different layout in a site so that one can not only style it but also do some limited A/B testing on certain design attributes of the site.
But, the CSS frameworks css files are so massive that one would not consider combining both the site css and css framework's own css in the form of LESS or SASS files. So we get this Hyde Jekyll thing where some vars are changed via the LESS or SASS files and having to use extra css code to get the site html files to a state where its update-able.