Linux, mainly.
Let's think of a setup where you have computer in another room, and in the living room you have a TV as a second screen for the computer and a remote controller to control VLC, but no mouse. When you need to do something with the mouse/keyboard, like load subtitles from a file, it would be incredibly handy to have at least the VLC controls in the screen that's in the same room with the mouse.
Well, in Windows this is a fail setup anyway since you'll constantly get windows opening in the mouseless screen (and need a hotkey to bring them from there), but in Linux you can configure the displays as separate, so that you can't even drag windows between them. So the clone filter able to clone the video to both displays would be the optimal solution here.