Hey, zurtito!
It's usually used together with some tool to make the front-most screen the default one, like the one I'm using for OS4,
DefPubScreen v3.00. Shanghai bit will, as Piru says, open in that 'promoted' screen any window that would open in WB screen by default. It's very useful for example in IBrowse when used in its own screen, when using an external editor (I use Notepad), it will open in IB screen instead of opening in WB screen, or a CLI window, etc.
Saluditos,
Ferrán.
