My side monitors are oriented vertically, so if I put those windows in fullscreen and try to share the whole monitor it doesn't come out correctly proportioned. This also provides a smoother experience without the "flicker" of going fullscreen and back (which can happen when you switch focus between windows).
It's a similar use-case to the one bentcorner described above.
The "share region" solution is admittedly a bit of a hack but it works. Love to find an easier solution to this scenario (especially a way to break out speaker notes into a floating window I could position right under my webcam).
BTW apropos to nothing, I always loved the way the old Spy++ tool let you drag a reticle onto a control hWnd (reminiscent of how "Select an Element" works in Chrome DevTools) - that was a neat mechanism to quickly select a perfectly carved out region of an app without having to draw a box, but alas these days with so many going Electron, and other non-standard presentation surfaces, it wouldn't be as useful anymore.
