We use it to notify our upgraded users with emails or forum notifications when they receive a new message in their exclusive threads (we do personal consulting in these threads). Since XF default behavior doesn't keep notifying users about a new message in their thread if they haven't viewed the latest comment, many of our messages were missed and the service we provide was not that great. This add-on solved this issue for us, so upgraded users will always receive notifications regardless of their thread viewing status. It's even better that it could be activated per a specific user group and/or nodes.