This is something that has been asked, and explained, and asked, and explained, but I thought this explanation was a good one:
"I'll give you an example of a theoretical expansion.
Let's say we wanted to do psionics. We'd tie that to a campaign you can play, maybe one centered on mind flayers or a similar foe.
The psionic sourcebook would be the player's companion to the DM's mind flayer campaign. The sourcebook would have all the info for creating psionic characters, along with world material for players who are creating characters for the mind flayer campaign. The player's book might also have a chapter written from an in-world perspective on psionics and psionic monsters, the kind of information that a character might have access to or have heard.
You can expect us to do one or two such products a year, to give people enough time to play through a campaign without overwhelming them with new options."
