This is speaking speculatively but Facebook right now is heavily trying to go into Messaging AI (Messenger) and so it needs to train those AI. I'm actually kinda surprised if they foresaw that training data so early but yeah it's really creepy.
It's not hard to foresee wanting "everything" with the goal of sorting it out later, but Messenger is a no-brainer for FB if they want to continue having access to message content.
