It's basically (when run the traditional way) a means for the press corps to apply pressure, if an issue is either emergent or common, a substantive question is likely to be asked and answered on it, and followup can occur on other forums, which are less public and provide less pressure on their own, but far more fertile when the issued been raised based on the public questions and responses (or lack of response) and stories based on that are out or imminent.

OTOH, if you bar outlets that tend to ask bothersome questions from the briefing in favor of friendly outlets, that breaks down.