I see now you've said that I've not made myself clear. (ref Belimawr)
Whilst certainly I can start a group, I am then forced to spam guild chat for a while until I have all the required members, or join the dungeon queue at some point. Often times it is a case for me of 'is this happening now?' or 'is the group starting, but the dungeon an hour away?' and there is nowhere to indicate that kind of information so it must be discussed. To debate it in guild chat is fine if it's quiet but as I'm sure you know, if there are one or two other conversations in flow it really isn't convenient.
Other solutions might be vent, but that is not used widely enough, and individual whispers which between 4 other people and possible more contending slots it is just a pain to manage.
Having this channel will filter all guild chat regarding looking for dungeons, which often in my view gets lost in the stream of other chatter and makes reams of whispers, or premature group joining/dropping unnecessary.
This is nothing to do with not being proactive enough to start a group, but a way to facilitate those proactively starting a group.