Following are excerpts from an article on Treehouse blog
Fixed Headers and Navigation Bars Used in Web Design
The trend of fixed heading styles in web design has grown exponentially in recent years. I have read a lot of criticism, both for and against these techniques, and generally it boils down to personal preference. Lots of designers have a problem dealing with navbars overtaking their screen with little-to-no value added into the user experience. Others find these techniques immensely helpful, as they provide quick access to popular links without the need to scroll way back up to the top of the page.
The first and most obvious interface is to apply your navigation bar settings so that they look natural in the page. This means you can setup a layout where the top navigation appears to blend naturally along with the other page elements. Take the example from Theory’s website.
Another good reason why different sites will use this style of navigation is for easy-access to important webpages. Even if there are lots of other sub-pages you can still point out the most common ones which gather the most number of pageviews. This can help first-time visitors to gauge the type of content on your site and how much interest they really have.
Often times I am surprised to find this design style in so many layouts. Using a fixed heading panel is a unique way to setup background images or videos above-the-fold of your website. These could also be vector illustrations, your company logo, branding, really any webpage elements to catch the visitor’s attention. The homepage of Google Ventures is one example.