Rocket.Chat is a great chat app for a "go to have a chat" experience. Users need to specifically want to go into the chat section to engage in talks.
Jot Messenger is a direct messenger, fully integrated with UNA profiles and pages. Much like Facebook messenger for example. User doesn't need to think about going anywhere to engage - its a direct contact, as long as they're on site.
They do have plenty of overlapping use cases though. As we develop Jot more it would reduce the need for Rocket.chat even more.