This would be useful for any group of pages that can't be added to a category. It's basically a request for a way to create watching links, where the page creator can define which pages are watched from that link - allowing variables like %%children{pagename}%%

For example, a forum thread with children pages used for replies. (watch a thread for replies)

Another example: A page for a PC application, with children pages used to represent different versions of the application. (watch an application page for future versions)

And yet another example: watching the system:members page as well as all pages in the profile:something category. (watch for new members on a site by clicking on one link on one page, rather than two links on two completely different pages)

And to watch a blog: Add a watching link on the home page, instead of forcing people to visit the blog category first.