You may be thinking what is the Navbar and why do you may need to turn it off?
Well, it depends on your blog's purpose and style, but the Navbar is a navigation bar that lets visitors move from your blog to other Blogger.com blogs. It looks like the second screenshot below.
Of course, it's up to you if you'd like to keep it. One plus for keeping the bar is the "Share" function so that visitors can share your blog.