Unlike previous versions of html and xhtm (differences W3C) where the code gave limited structure which was determined by the heading tags <h1> - <h6> tags, html5 (W3C) provides much more structure.
All of the layout tags for html5 are semantic elements that determine the structure of the page. Each tag describes the type of content and its relationship to other content. Whereas before you could use divs indiscriminately the new tags require that some thought is given to their content and placement. I would imagine this will become especially important in terms of compliance to section 508.