Merely as an example, I remember once, some time ago, that I had systemsettings open at kcm_colors customising a theme, I did something else, forget that I was, saw systemsettings uselessly open, and closed it. This closed all my customisation windows.
I’m aware that a solution for application developers is to implement a warning dialog for all destructive actions, but I’d like something at the WM level, since very rarely does a window have children which aren’t of any importance whatsoever.