I'll give another example with mixins: clearfix. I've seen webpages littered with class="clearfix" all over the HTML. That doesn't do anybody any good. With a CSS preprocessor you can define a clearfix mixin and just write it something like:

.class-name .clearfix font-size: 1.2em

There are other ways to do it like one long string of selectors followed by the clearfix code, but they're not as convenient or readable.