Your company has a user pool, you sign a BAA or start working with a partner company that has their user pool. Instead of creating slack accounts in both you can share external slack rooms that only people that are invited in/from their respective orgs can join without having to co-mingle employee user pools.

But why would external partners want to look at your code? I guess if you're also integrating with them? But generally you just give them repo access instead. For Slack, it's different as messaging is a core feature to collaborate between different people in different companies, but looking at code is a very specific use case.

Not sure, I was only answering in regards as to what Slack shared rooms brings to the table for companies in the form of letting Project Managers/Account Managers have direct line of contact with clients.

Code wise I guess you can could be working with any agency or contractors and you could collab on PR reviews? No idea to be honest.