Shared channels in Microsoft Teams create collaboration spaces where you can invite people who are not in the team. Only the users who are owners or members of the shared channel can access the channel.
While guests (people with Azure Active Directory guest accounts in your organization.) can't be added to a shared channel, you can invite people outside your organization to participate in a shared channel by using Azure AD B2B direct connect.
You might want to use a shared channel if you want to collaborate with a group of people who are all members of different teams.