This is a great question. On the old forum, we had "Visitor Messages", it's a bad name, it sounds like an out-dated guestbook.
On the new forum it's called "Profile Post", still not a good name but better. You can use it to update your status, like on Facebook. As you know, social networks have been very popular and taking over the Internet, xenForo has been trying to make the forum software more social by learning from FB.
You can post anything you like as profile posts, it's your personal space, you have complete control over your profile posts, delete or edit without time limit. For example, you can post something like:
I don't feel like posting today.
Or
I will leave town for the next two days.
Or
I'm out of here. (no, not this one)
Such posts may not be appropriate to be posted as forum threads.
So, anyone who cares about you can know why you haven't posted by checking your profile page, like checking a friend's FB account.
Many xenForo forums disabled profile posts because their members didn't know how to use them, after I saw
@hollydolly used it perfectly, I was encouraged and added it back to the sidebar.
Please note:
1. Profile posts are public, do not use them as private conversations.
2. You can post anything you like, but if you want other members to participate to make a hot topic, then you should post it as a forum thread.