What’s the reason behind fixed (position) navbars?

It's becoming almost customary for responsive designs to have fixed-position navbars, where the navbar is fixed to the top (usually, or even bottom sometimes) of the viewport of the browser.

Don't understand what I mean? Take a look at Twitter Bootstrap's project page. There's the dark navbar at the top. Now scroll down. You'll notice that the navbar doesn't go away. It stays, affixed to the top.

It's not just Bootstrap. Many large websites have been switching to responsive design lately, and most of them (like 90%) have incorporated fixed-to-top navbars. Some examples include: Mashable, ReadWrite, TechCrunch (not responsive design, but still), etc.

The question is, why? What's the reason behind this UI decision? Does it have any prominent advantages other than providing quick access to the menu?

Personally I find it very obtrusive (on desktop / laptop -- it's more natural on touch devices). And is there any study stating that the fixed-to-top navbar is intuitive?