Talk channels can be set up by managers or default talk channels can be set at an organisational level by an account owner. In this case they will then automatically appear in any new course activation.
Broadcast only talk channels are a useful tool to make announcements as only facilitators can post messages, but learners can respond.
Learners can also enable email notifications to make sure they don't miss out on any announcements.
Other articles you may find useful:
How to set up default talk channels
Overview of talk channels
How to enable email notifications for talk channels