When audience campaign is set up, user data will be transferred to Facebook as per defined schedule - either once (for one-time campaigns) or as per defined interval (for recurring campaigns). When running a recurring campaign, the new audience overwrites the one from the previous upload.
In case Facebook campaign runs in real time, the users will be added to the segment on Facebook immediately after the journey is fulfilled.
The benefit of running a real time Facebook campaign is that it allows to target users right after they pass through the user journey. However, the relevance of such real time campaigns is sometimes limited and highly depends on your use case.