Chances are, if a monitor sits on your desk, more than one device is plugged into it. Maybe your setup is a personal PC and a work laptop, or a computer plus a gaming console. Perhaps it’s all of the ...
My cheap-ish Samsung (cheap for a QHD) has an HDMI and a DisplayPort input. If I hook up a laptop to each input, both laptops think the screen is on- even the one who is not currently being displayed.