The reason why it's interesting is that it helps Facebook to avoid a big problem with any platform (aside from the web, of course): people are really lazy about updating software. The higher into the tech elite you go, the less true this is, but for the vast majority of users, updating software is something that becomes habit to ignore. Google Play does offer automatic app updates, but that feature isn't on by default, so the ones who really need it, don't use it (unless the option is toggled by that person's "tech savvy" friend or family member.)
The idea is that Facebook would be able to avoid this trouble, and offer the app updates directly to users automatically, which would make it much easier for Facebook to ensure everyone was using the same software. The updates would only be downloaded over WiFi, and won't use your data plan.
There are a couple troubles with the idea though. First, it's unclear if Facebook is even allowed to offer an app with this sort of functionality in the Play Store to begin with. The only other apps that we can think of that do this would be the Amazon Appstore, and Swype, and neither are offered through the Play Store. And, would Facebook be able to get enough users to get the app outside of the Play Store. More importantly, this presumably would have to be done through sideloading, which opens up strange security risks when that option is toggled on the device of someone who doesn't really understand the implications.