For one not-minor advantage, you get context to badging and push notifications. I have the notifications from the Facebook app itself completely disabled because it's a flood of non-urgent interrupts—but I rely on notifications on the Facebook Messenger app, and would probably leave notifications enabled on Facebook Groups if I had a group I participated heavily in. It'd be sort of like Pair, or Slack (but for non-company groups), both of which are disruptive sub-markets that are eating Facebook's lunch.