As websites grow in size, so too do CSS files grow in complexity. Even a relatively small site can have a dizzying amount of CSS declarations demanding greater effort to understand as time moves on.
CSS Pre-Processors simplify your CSS through programming-ish functionality like mixins, variables, conditional statements, and iteration.
In class we will be working with the Node.JS-based Stylus. Others like SASS and LESS share similar concepts, but different syntaxes. The newer Rework promises more flexibility and modularity, but I suspect stylus might be easier to begin with.