The main purpose of CSS submission is to enable the separation of document content from its presentation such as layout, colors, and fonts. This feature of CSS enables better crawling of the web page also with the help of CSS a table less web design is possible.
CSS can provide some compelling SEO/SEM benefits. These include:
Moving important content -- especially the unique text that you want the page to rank for, along with any related links -- to the top of the HTML file. This is probably the single most important reason for using CSS from an SEO perspective, and can have a significant impact on rankings.
Making pages "lighter" in weight, and decreasing the code-to-content ratio. Done properly, this can make your entire site easier and more efficient to crawl, leading to more and better indexing of your site's content.
Using standard HTML tags, such as <h1>, which search engines recognize as indicators of important content, instead of fancy <font> tags
Using standard HTML tags, such as <ul> and <a> to construct menus that contain standard HTML links to other important areas of your site. This makes these links easily discoverable by search engines, helping ensure more efficient and deeper crawling of your site.