When meetings are covered by media then people tend to play to the press -- they also often focus more on "winning" looking good or will avoid saying things that they can say privately.

Examples:

"Hey, I suggest John Doe could lead this project."
In private someone could say "No way, that guy is a scammer" -- one might resist saying an accusation in public because they don't want the fight or are not 100% sure

In private I can say "Hey, Jane, you've really upset a lot of the developers with the tweets you made accusing BIP 200 of being of bad intent...lets talk about your concerns now and try to avoid that in the future"
In private I may not want to embarrass Jane or may want to avoid an unproductive flame war

In private a CEO can say "If XYZ doesn't change in the code we are going to have to pivot and fire 14 employees and focus on project 123 instead."
Saying so in public can cause employee panic etc.

People have concerns with investors, potential partners and so on.
Just like private conversations in a hallway...lots of advantages of privacy.