To manage abuse of the chat system.
I am not sure if admins can see personal chat of a user (I haven’t tested it out yet to that level) but with open chat channels, abuse will be spotted by the users becasue it is essentially public.
My thinking is along the same line of this idea pre-chat, to have it in both DM’s and Chat would be consistent and give admins good control over nefarious activities using Chat/DM systems.