PM's are now "Conversations". You will now notice when you send and receive "Conversations" its very similar to posting as you have a complete field to work with. You can easily add photos as well.
To answer your question you have the opportunity now to invite others to join into the conversation. So a good example is you want to talk to more than one moderator or member in the same conversation. Just add each participant and there you go.
Example. You want to include me in a conversation with you another member. You will use @Roll_Bones and the other member @example and now you have three participants in one conversation.
This is "Optional" and private "Conversations" are still private.