The success of Apple's App store, among others, has ensured that many large companies will try and create their own App Stores too. Some end up being major successes, and others aren't useful to people at all. Besides Apple's store, one of the most popular digital stores is Steam, an App that lets you buy and download games, without the worst draconian DRM, reviews, and other built-in social features. There are other gaming marketplaces such as GOG's Galaxy, that also have some nice advantages such as no DRM, but Steam is obviously the biggest player here.
Facebook obviously wants to create features that not only ties you to their platform, but keeps you interacting with your friends longer. Gaming is one of the best ways to accomplish this goal because a dedicated gamer can spend hours per week (or even day) with their favorite game (and Facebook wants visitors to spend lots of time on their site) and spend lots of money on addons and in-app purchases. This kind of marketplace is especially conducive to the casual gaming that dominates many mobile app platforms today. Casual gaming is a great market that has both positives and negatives: it expands the audience for gamers and makes it easy for more people to play games, but on the flipside the games aren't always as deep and sometimes rely on bad play mechanics to entice people to spend lots of money on coins, gems, stamps, dilithium crystals, or other items to speed up gameplay.
While this App will undoubtedly be focused more on casual gamers than Steam is, if this App platform can strike the right balance between deep and casual gaming, and be focused on making money through in-app purchases without being too onerous of a money grab with substandard games, then there's a chance that it will be very popular due to Facebook's massive userbase as well as potential for deep social features within games that even established networks like PSN, XBox Live, and others can't come close to matching.