One benefit is that the styles only need to be downloaded once – on the first page the user visits. The stylesheet goes into the browser’s cache, and thus need not be downloaded again for other pages that use that particular sheet.

Another factor that speeds up browsing of sites with separated style and content is that shorthand is used to describe a set of styles. A page that has, say, 20 occurrences of ‘ ‘ could replace those with 20 ‘class=”important”’ parameters. The more tags that repeat across the pages of the site, the greater the size savings by using CSS styles.

Source: Wikipedia Style