Supplements for the GM on the go, one might say. We've all been told by the designers that there is not a lot of money in RPG design, and what money there is in RPGs is not in the core books, but the supplements. Because 'the money' is in supplements, it is a rare game which has none. This is all well and good, conceptually. I have zero problems with adding depth, new system ideas, or completeness to a game.
Most designers play in the universes they create.
This seems only fair, after all, I'd want to play with something I spent time developing. However, quite a few designers want to share the world changes that happen in their games, they want to share their stories. The problem is, most of their stories are not congruent with the games that are being played across the globe. Then along game Forgotten Realms second edition, in which the pantheon of the gods changed. That was no problem, we simply ignored all the changes sticking with all our first edition stuff.