• Many designers have shunned the "h1", "h2", etc. tags because, well, in their unaltered state, they're generally pretty ugly. Trouble is, most search engines depend on these tags and others like them to prioritize information—something in an "h1" tag is theoretically very important, and neglecting these tags can mean missing out on keyword relevance as measured by search engines. Enter CSS. These tags can now be styled to look however the designer wants, while still maintaining the formal structure in the HTML file that helps the search engines do their jobs.
• As Frank mentioned, CSS helps separate content from layout. Not only does this make the file easier to read and modify, but it means that your file will have a greater proportion of relevant keywords (as opposed to utilitarian markup tags). This will make more of a difference for some search engines and less for others, but it won't hurt, and could be helpful.
• Just as CSS styling can make the previously unfashionable "h1" tag useful again, CSS can change the way things in the file are rendered on screen—even the order in which they're displayed. Some search engines decide what is most and least important based at least partly on where keywords appear within the HTML file—my optimization program emphasises the opening paragraph and closing paragraph particularly. However, sometimes I want something different at the top of the page (or the bottom; think the boring footer information). With CSS, that's no problem. I just reorder the layout with CSS and then I'm happy with the layout, and the search engines have a good idea what is relevant in my copy and what is not.

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