It works really well for scaling your systems. In the case of "third party" this can mean disparate services within your platform, permitting you to fully isolate the authentication from literally everything else.
While they do not necessarily use consent for delegation, they do use automated policies to restrict API access to what the application needs.
Likewise, having a common protocol for getting authentication and API access allows them to decompose systems. Having to build first party authentication and API access systems would naturally encourage applications to become monolithic.