If you're wondering why app developers will support the new Sign In with Apple service, they'll have to, if they offer any third-party sign-in options; from Updates to the App Store Review Guidelines | Apple: "Sign In with Apple will be available for beta testing this summer. It will be required as an option for users in apps that support third-party sign-in when it is commercially available later this year."
"These didn’t come out of nowhere: Apple has been playing up its privacy game for at least a few years now, and in the Tim Cook era it’s especially come to the fore. But today’s announcements really crystallize how Apple’s approach to privacy will mesh with its transformation into becoming even more of a services company. It’s becoming a services company with a key differentiator – privacy – and it’s also extending that paradigm to third-parties, acting as an ecosystem layer that mediates between users, and anyone who would seek to monetize their info in aggregate.Apple is now the privacy-as-a-service company | TechCrunch
Apple’s truly transforming into a privacy-as-a-service company, which shows in the way that it’s implementing both the new single sign-on account service, as well as its camera and location services updates in iOS 13. The SSO play is especially clever, because it includes a mechanism that will allow developers to still have the relevant info they need to maintain a direct relationship with their users – provided users willingly sign-up to have that relationship, but opting in to either or both name and email sharing."