To properly optimise your web site, you must consider search engines, download times, linking stratigies, keywords, robots and spiders. If your site is written entirely in HTML then it is no doubt bloated, out of date and created without any thought to search rankings. It probably uses old coding formats that have been depreciated and ignored by Yahoo and Google.
Think about your own browsing habits. How long will you wait for a page to download? How many pages of search results to you check? Your viewers feel pretty much the same way. Brevity is the soul of wit. Well, CSS may not be witty but it sure loads quickly and helps to allow search engines get to the essence of your message. When you use CSS all that old, bloated code can be removed leaving only the content in your HTML. CSS applies styles and formatting to your pages site wide. You only need one file! Put your site on a diet!