I'm assuming by "using the window manager in Linux" you mean you can manually put windows always-on-top.

Three reasons this is different: 1. The window is actually not on top of every window - it's only top of your browser. It's "attached" to your browser window, same way a real PiP would. 2. It's less clicks and also positions the window automatically for you. 3. It's cross platform. Windows doesn't have an easy user-accessible way to enable always-on-top.