 <h2>Flexibility</h2> <p>Since Bootstrap is a library, they realize that some people will not want to use classes from Bootstrap, but will want to use the mixins. By keeping the separation, a user can import <code>mixins.less</code> and apply <code>.clearfix()</code> into their own code under their own class name(s), <em>without it actually creating an output of a <code>.clearfix</code> class itself</em>. At the same time, if one wants to use the whole Bootstrap library, they want to provide an actual class by the name <code>.clearfix</code> for users of the whole library.</p>