Work does not involve collaborating with people inside your organization only, but with people outside your organization too. It becomes tedious to constantly be in touch with them, and share resources with them, and more tedious when it’s not just you and them but involves teams on both sides as well.
Shared Channels allow you easy collaboration with the people outside your organization inside Channel. Both teams need to use Channel and they can send messages, and share files in a single channel. Thus they can be connected and work seamlessly.
And this does not compromise your organization's information too. The team invited to a channel can only access that particular channel, its chats, and files, and cannot communicate with others in your organization or access other channels.
So now, you can have both flexibilities with access control at the same time.