Having an application for approving access packages (policy) makes it possible to build in custom logic/rules for allowing b2b users accessing the tenant without user interaction. E.g for an MSSP that works 24/7 can request an access package and trigger an application they own to approve the request. The Application can check for further internal conditions whether the requester should have access or not. Internal condition be a lot of thing, but some of them are.
* IP from the office?
* work time schedule
* which access package is requested.
During the night you can't rely on an internal sponsor to reply on a request. Having external sponsors works, but is not scaleable as in a business people will always come a go and having them replicated as b2b guest user over numerous tenants only results need a lot of work and manual synchronization to maintain the state you want.