Its many advantages.
Cascading Style Sheets
CSS or Cascading Style Sheets is a feature used in HTML that allows a designer to control how information in formatted on a page.
Using CSS we can create a style sheet that defines how different parts of a page are supposed to appear. Then the browser will apply this information to the page when a user looks at.
Another aspect is that you can use a different style sheets for and change them out without rewriting the entire page. This allows you to create a site that can be good looking for every different type of user (users with different monitors, different browsers, even users with visual problems).
For a great example of this in action, check out the CSS Zen Garden and load different designs onto the same page.
ZonerDesign, a web designer from Queens, has put together a list of CSS sites. You might find a name you recognize about a third of the way down.