There are a few higher-level goals, but generally speaking, it boils down to two main advantages. For admins, they don’t need to configure and maintain Github, Google, Apple, Facebook logins. Each of these on its own is not too hard to set up and maintain, but it’s multiple steps as well as maintenance. And in Apple’s case, cost.
For the end user, there is not a huge difference at this time. In the long run, if enough communities adopt Discourse ID, we hope to offer a better centralized interface on ID (for the communities a user joins) controls for notifications, customization of user accounts, etc. And better discovery for new communities.