One of the advantages of using CSS to lay out and style a web site is that you can begin working on the content and structure of the site before you ever pick a font, background color, or link color. This can be great when you are working with a client who is focused in on the way the site will look early in the design process while you are focused on doing a content inventory and developing the Information Architecture.
It’s pretty easy to explain how you build the site starting with the content and then formatting the pages based on the content of the site using CSS to even a non-techie. The benefit is that clients, who aren’t generally interested in how you code a site, can understand when you say that you want to get the content together and figure out the structure of the site and then style the content using CSS to get the desired look and feel. This can help get clients focused on the unglamorous body copy and images that they need to help you put together early in the project.
One of the hardest things to do is convince people that want to see fancy rollovers and pick color palettes that they should focus on the text of the site instead. Copy writing is simply not as fun or cool. So, by explaining that building with web standards requires you to work from the content out you can redirect their energies and produce a site with better content that will also be usable and accessible.