[mushishi]

I would say they help to scope a part of a discussion that clarifies some detail or maybe when giving a link elsewhere, without derailing the mainline of the discussion.

It's useful when people arrive in the discussion at different times and you don't want to necessarily let the whole channel to be notified about your specific remark, and don't want to write a private message either -- one reason being it would lose the context.

In our project we have multiple channels that have actively 3-5 persons and work at different times. The things we chat about are lightweight changes or news, or maybe someone has a quick clarifying question about implementation detail. Something that wouldn't be posted in an issue for one reason or other.