If you're on this page you probably already know why having a sticky navigation bar is really important. But just in case you don't, let's suppose your visiting a website and you are on a product page. You scroll down past the photos, the description, the product testimonials, reviews, cross selling and recently viewed products only to decide that this product doesn't interest you. You'd like to see what else is available on the website.
At this point, if you have a sticky bar your website's menu is within one click, however without a sticky bar your customer will have to scroll all the way back up to the top of the page which can make or break their decision to stay on your website. You might get away with this on desktop but on mobile it's an essential feature.
If you have long pages on your website then you need a sticky navigation bar.