You're asking that as a manager or employee? If the former, I suspect there were channels you didn't know about. If the latter, I don't know - but maybe they were channels you didn't know about :).

But the reason is, these chats are for conversations you won't feel comfortable having with your boss around. Cat pictures, stupid jokes, random topics. Even if people felt comfortable with such topics around their manager, they might still be worried that the volume of communications will be seen by the manager as evidence that people are slacking off.