Looking it at the source, -s makes gedit always start a new process and a new window, rather than adding a tab to an existing gedit window that's already running. Useful if you're worried about one window crashing taking down the other. You can do the same thing with gnome-terminal with the "--disable-factory" switch, which is useful, as g-t has been known to crash, and take down the processes running in all your tabs and windows with it!
Edit: And the docs you linked above point out that it's also useful if you're invoking gedit from a batch file, or from a mail program, etc. - "gedit -s" will create a new process that won't exit until the window is closed, whereas plain "gedit" will return immediately if there's already a gedit process running.