I sometimes have to take screenshots of video streams. Grab/Screenshot is very good because it keeps the selected area at one and the same place. So if the app remained on and I had placed the selected area exactly where the video stream is on my screen I would just have to press Capture to take screenshot after screenshot.
I'm aware of the shortcut function but, unless I've missed something, that requires me each time to select the video stream window, which causes me to miss the important parts, like people's facial expressions etc.
Hence, why I think it would be good if Grab/Screenshot would be active until turned off. Incidentally I've noticed on older versions on OS X and macOS that it would sometimes, but no according to any discernible pattern, remain on to allow for a second screenshot. But that does not appear to be the case under Catalina. I guess I should submit a feature request.