I think it causes the same notification preferences to be visible on a staged user’s profile as are visible on any other user’s profile. So you could, for example, go in and set a staged user to be watching a category.
One obvious use case would be that setting staged users to watch first posts in a category and posting announcements in that category would provide a newsletter experience.
Sadly the user preferences don’t list topics watched so you won’t be able to use that setting for your purposes.