The <nav> tag is introduced by HTML5 which defines a section of navigation links. Not all links of a document must be in a <nav> element. The <nav> element is intended only for major block of navigation links.
The major search engines such as Google and Bing will see NAV as containing the structure of your website, and you will get an ACCURATE extended listing in the search engine for your main site entry.
The NAV tag states clearly that these are the main pages within the NAV tag. Otherwise search engines such as Google may randomly choose to list different pages.
It also helps with BOILERPLATE repetition. Search engines such as Google know this is a repeated element and will not penalize your page rank for duplicate content.