One way they help is by putting all of your formatting, styles, colors, etc in one easily changeable file. You can, for example.. have one file that tells your whole website how to look.. Need a new look for your website? You need only change this one file and viola.. your whole site looks different. Colors of links, text, how big fonts are, what fonts to use, what color to make the background… all sorts of things can be controlled by this one simple cascading style sheet. See this introduction for more info about it.