People post to wrong channel all the time. On large project, with many users, this creates chaos, and people miss important messages. Be able to go back and move to the appropriate channel.
Should use mouse, to allow selecting multiple messages at once, and move them all.
Admin should be able to do this for any message, and author should be able to do this, and channel should have permission setting where admin can enable anyone in channel to move messages out to other channels they have permission to post into.