Suppose I have a following situation. Long ago I published some useful code on Github and a lot of people forked it since then. Now I find some really serious error (like a buffer overrun) in my code and fix it and I realize that all forks should better have that fix, otherwise Bad Things™ might happen.
How do I notify owners of all forks that there's this critical change they'd better pull?
An upstream repo doesn't really know about its downstream repo (see "Definition of “downstream” and “upstream”").
And you cannot make a pull request to a fork (that wouldn't scale well anyway).
So the easiest was is to count on the other developers to update their local clone with your latest changes, which will include your latest fixes.
You can update your README.md for all to see, but you cannot really "broadcast" to all the forks (not to mention all the direct clones you have no knowledge about).
Anyway, if they want to contribute back, you will reject any pull request which isn't fast-forward.
That means they will have to rebase their work on top of the latest from "upstream" (your repo), before pushing to their fork and making said pull request.
