Simply put the app adds the top bar that becomes visible when the customer scrolls below your Buy button. The bar contains the Buy button so user can quickly add your product to the cart without scrolling back up.
Basically because you want to make it as easy as possible for your customer to make a BUY.
Amazon use it. Check it out for yourself. We're not saying you should as well just because the e-commerce giant does, but trust us if you wanna increase conversions, you should :). It's a no brainer!