Facebook is always after more money to help the social network operate. I am sure that with all the people that use the site the costs of maintaining are astronomical. Facebook certainly has a massive stockpile of cash on hand so it's not hurting, but more money is always a good thing. Facebook has added in new in-game offers to allow developers of games to grab more money.
This is the same way that in-game offers on many Android Market games work. If you are playing your favorite Facebook game, and you need more credits or something else to get your game on, you can buy it right in the games via tabs. The player can also get free credits with the in-game offers by downloading and installing other games and apps.
In the past, only the largest developers of games were allowed to do this sort of thing on their Facebook games. With this move now even the smallest developers can have Facebook Credits and are paid out in whatever the developer's native currency is. Naturally, Facebook gets a cut of the money, 30% to be exact.