One reason would be to use a standard PC screen and standard Office software with a teleprompter. (I find the whole business of teleprompters a bit arcane, it seems to be either, professional and massively overpriced, or cheap and cobbled together).
The other reason is to display your screen in a mirror, I can’t think of why you would want to do this but I’m sure there are some marketing and artsy types who might need it.
Anyway here is some software to do it – Ultramon. It’s $40USD https://www.realtimesoft.com/ultramon/