If you have a phone from Apple, you get your apps from their App Store. If you have an Android phone, you pick up apps from the Google Play Store. Pretty simple, right? Well, Facebook is concerned about not being able to distribute its apps via these methods in the future, so they're building their own app store.
According to rumors, Facebook is concerned about Google's ability to pull their apps from the app store on Android devices. Since Google has the final say in all matters regarding what can be offered in the official store, they can technically pull apps for any minor reason. And for some reason, Facebook is very concerned about this. So concerned that they're making their own app store to deliver their apps to people, should this happen.
Remember back when Facebook removed the Messenger functionality from their flagship app, and moved it to a new one? People hated this move, as it forced them to download another app to get the same functionality. I bet people are going to be thrilled to have to install a completely new app store, just to download Facebook and its Messenger app.
If Facebook really is developing their own app store (which is completely different from the Oculus VR App Store) out of fear that Google will remove their apps, then there must be some other force at work. With all of the other companies out there that Google competes with, how many have been blacklisted from the Google Play Store for no good reason? Either Facebook is planning something big that will upset the search giant, or paranoia is sweeping the upper levels of the company.