You used to be able to connect to facebook messenger via XMPP. Combined with this permission, it would have let you retrieve historical messages and add persistence among alternative clients.
If Adium, Pigeon, or another collective chat application needed that permission at the time to combine my facebook and AIM lists into a single application, i would have certainly understood it.
In late 2013 I used (I think) the Graph API to pull my own private chat messages so I could see what messages I sent and received on a particular day, to remind myself when certain events happened. I can't think of many good reasons for this API to exist to third parties, but it was pretty handy for my own data.