It would allow Zuckerberg to talk more candidly. Public perception and opinion is very important to any of these billion-dollar business leaders. If you have an open hearing then there is a snowball's chance in hell that you will hear anything that wasn't carefully planned by teams of PR guys and lawyers.

A closed hearing won't be picked apart and (mis)interpreted ever which way, so the speaker can try to get their true point across. The speaker can also acknowledge faults and mention some sensitive issues without making their company's stock ride a rollercoaster.