Private shared channels are basically an expansion of shared channels, except they are…private. This means you can choose to hide the shared channel from certain users in your team, and only let a select number of members access the shared channel. These shared channels can be set as private in one workspace for one team, and as public for the other, or as private for both which is pretty clever.
Shared channels are basically a supercharged implementation of guest access that’s super useful when your company or team needs to work with a partner company or an outsourced team.
Slack is starting to put some pressure on Skype For Business I think anyway.