Specifically for Composure and virtual production, it would be very handy to be able to be able to go into a clean fullscreen for the preview windows you can create. This would allow you to monitor a comp with final output color with really low frame delay and not go into play mode.
If I make an output through my capture card, the frame delay is significant but I can operate without going into play mode. If I make a viewport output and move that to another display, I can have low frame delay and fullscreen but I have to be in play mode. If I was able to move a preview window to another display and fullscreen, I’d have low frame delay and not be in play mode.
This is the same for both UE4 and UE5.