What are WAI-ARIA roles and what are they good for?
WAI-ARIA roles get used to annotate page elements. One of the goals we want to achieve by using WAI-ARIA roles is to improve accessibility, this means improving the way screen readers parse the document and therefore make it better machine readable so that screen readers can represent the content to the user with text-to-speech, sound icons, or on Braille output device.
Another goal is that we also improve the way search engines consume our content, because if you care about SEO it is never a bad idea to improve the semantic value of your document. If you use the ARIA roles and maybe also the new HTML5 elements you will improve the semantic value of your content. The more we define the structure of our documents the easier it is for a search engines to extract the valuable content. We can therefore also avoid getting penalized because we are tagging the content that is complementary and that is found on each of our pages, by clearly separating it from the more important part of the document.