Before shared channels, your administrators had to give a guest account to any external partners you wished to work with in Teams. However, this might require extensive configuration to prevent too broad opening up your tenant.
Even after the fact that your users never need to leave your tenant when using Shared Channels, there are some pitfalls and dangers you need to mitigate. Your tenant may be exposed if you don’t update your governance policies to reflect and address the dangers involved. Collaboration is excellent, but you shouldn’t have to give up security to do it.
Shared channels aid in preventing unintentional oversharing and prevent the development of extra workplaces just to work with outside users. Despite this, this new function still leaves room for oversharing and sprawl.
The introduction of shared channels ushers in streamlined communication in a safe, controllable, and legal setting. With the help of this new feature, co-hosting an event, co-writing a white paper, or working together on any kind of outside program or project will be a breeze with less likelihood of forgetting logistics, having translation errors, or having technology impede cooperation or productivity.
https://docs.microsoft.com/en-us/microsoftteams/shared-channels