A quote from the Autodesk blog:

What happened to C4R ?

“”Collaboration for Revit has been rebranded and repackaged as BIM 360 Design. This may sound like marketing, but what it really means is a very smooth transition and more capabilities. Collaboration for Revit is not being discontinued or retired – it’s being renamed and refashioned as something better. Current subscribers will see the product name and icon switch to BIM 360 Design in their Autodesk Account.  New customers who want to start worksharing in the cloud will subscribe to BIM 360 Design.

Communicator is evolving

Communicator, the in-context notification and chat tool, will not be available for use with BIM 360 Docs-based cloud worksharing. We are working on replacing essential functionality in future Revit releases.””

https://blogs.autodesk.com/revit/2018/04/24/from-collaboration-for-revit-to-bim-360-design-what-the-revit-user-needs-to-know/

By installing Revit Communicator you bring a new and improved version of team communications back inside Revit.