I am no authority by any means, but I think the history of EVE has a great deal of influence on this particular matter:
During beta, more traditional "forum roleplay" happened a great deal. Since it was just about all players could do that would have any hope of surviving to actual launch, it was noticed. Then actual launch hit, and all of a sudden what you did ingame mattered; what you did on the forum, did not (or, at least not to the same degree). The incentive for "traditional" forum RP was removed.
So instead the IGS took on a new role: Discussion of what was going on ingame. While words could not actually destroy spaceships, they certainly could provoke, hint, or attract players into fighting. Many corps were at the very least what we would call "RP lite"; more dedicated RP groups were also more numerous. All of these flocked to the IGS as a central point for making announcements where they could be sure they would be seen. In this day and age news articles were also far, far more common than they are now; at the very least, you could expect a new news piece every week or so, "fluff" or otherwise. This discussion, too, was funneled into the IGS.
In time CCP embraced it in this role as well, using the IGS as a central place in which people could throw questions at Live Event or news-central characters and actual expect a response.