If I have got a question that can be asked on different sites I have to choose just one.
For example, I've just asked on
unix.SE this question:
I believe that I can ask just the same question on
apple.SE site. But I don't think that it is polite.
So the idea is to make it possible to share questions between sites. Maybe it will be topics with different colors or topics in special section on site that will redirect to original topic on original site.
I am sure that here is many questions on main SO site wich can be discussed with
Programmers and with
CodeReview and even