The Blogger Navbar appears by default at the top of every Blogger-powered blog. It is available in several colors, and is configured in the Template tab in Blogger’s interface. If you are using a classic template, you’ll see a menu from which you can select a color. For blogs using Layouts, just click the “edit” link on the Navbar page element. Blogger gives a more detailed explanation of this
here.
There are several reasons why I think it’s important to remove the Blogger nav bar but I’ll name my two most important reasons. First one being that it’s an eye sore and makes your blog look less professional. Secondly, when you click on the “next blog” button you never know what type of blog you might stumble upon. There are so many spam blogs with adult content or other information you don’t want young people to read. When you do block the navbar on your own blog, you prevent others from also easily leaving your site and accidentally heading to one of these spam sites.