You may want to be able to see what is (or would be) projected in another screen even if you cannot physically see that other screen.
For example, when I'm presenting a topic with my laptop, I'm using the Extended mode so that the assistance does not see all my desktop with a million shortcut icon but just a clear screen. Also it is convenient to use PowerPoint with the Presenter Mode.
The challenge is then to make a live demo which is usually on one screen only, meaning if you want your audience to see it, you also have to look at the same screen (typically a classroom projector). That can be a physical challenge...
That is the purpose of DesktopMirror.