Space and Project stream are exactly the same where there is continuous flow of communication in just one setting. You and your teammates can share any update in Stream itself for tracking and quick view of the activities. You may attach files, mention @username and share your itinerary to your team.
This can be another form of an open conversation, however, if you wish to have it privately, you can have the option to use the project chat as mentioned earlier. Anything that happens in within the Space and Project, the activities will appear respectively in the Stream.
Stream displays all activities in an orderly manner from the latest update.
And when you click on the "..." at the right corner, there will be options to choose from Pin item, Notify Users or Followers List.