Semantic markup insures that the structure of your document or web page is clearly delivered to the web browser and to search engines.
Using semantic markup in your web site will help your site be properly indexed by search engines, generating better traffic. It will also insure that visitors using older browsers, or browsers with limited support, can read and understand your pages. The use of semantic markup also greatly helps visitors with disabilities, particularly blind users using a web reader, to interact with your pages effectively and efficiently.