According to Facebook, messages will remain private amongst accounts, so you can use pass someone else your phone and not worry about them reading your messages. “The beauty of this new feature is that your messages stay completely private. Anyone else who shares the device will just see the number of notifications of messages that you have, but they will never be able to see the content of your messages.”
As pointed out by Facebook, the first time you switch accounts a password will be needed, but after that users can choose if future switches will continue to require a password, or if no password is needed at all. This might come in handy if you manage multiple Facebook accounts for your brand/business.
It might also come in handy if you’re sharing a phone with someone, like a family member. In any case the feature is now live so you can check it out by launching the app’s settings and go to “Accounts”.