Search Engine Optimisation
Search engines such as Google are increasingly relying on an analysis of page content to determine a page's relevance to user queries. Although the number of pages linking to a site has an influence, pages with good content can punch far above their weight.
One of the advantages of clean simple design using xhtml and css is that the content of the site is a larger proportion of the total page than it would be using old fashioned HTML layout techniques. The practical upshot of this is that search engines see your content as the most important part of the page. Add this to careful editing of the text to emphasise search terms and place them in important parts of the page, and the page can get much better placement with search engines.