It is true that most modern browsers/technologies recognise the HTML5 <nav> element as navigation and give it the same attention. But explicitly setting the role="navigation" attribute just makes sure that a lot more technologies can pick it up.
For example screen-readers and other technologies for users with disabilities are very rarely fully standards compliant (especially if they have to work all the way back to IE6 or lower!) so adding the role attributes explicitly always ensures that you cover all your bases for the most users possible.
Also (and this is just a guess) some of the lesser known search engines may not fully recognise HTML5 yet, so adding these roles should help with the sites crawl-ability.