During large events the last thing you want to do is make it harder for users to get in or harder for the server to deal with the users. ConferneceRoom has had huge events of over 15,000 users in one room. If you were going to download the names list of the room and you weren't using Auditorum Rows then you could have up to .5Mb of information just in users as you join. This isn't practical as you're really only interested in those that have the ability to speak.
To solve this problem the channel mode +A was created. This mode called 'Auditorium Mode' is designed to handle large events without flooding everyone with unimportant information. It will show up to 20 users and all the users that have the ability to speak including both channel operators as well as those users that are set +v (voice).
After an event when the room is set -A the users will start to appear as they speak unless there is a small number of users, in which case they will appear to mass join the room.