Because the Internet needs standards to work. If all domains implement authentication differently, we do not have an interoperable network.
As is customary, standards must evolve to keep up with requirements. OpenID 1 and 2 weren't built with the idea that smart phones would be in every persons pocket, or Internet connected devices in every home.
The latest iteration of OpenID--OpenID Connect--is essentially Google's playbook for authentication. It's of huge value to the rest of the world.
To put it simply: having your own OpenID Provider at your domain (e.g. idp.example.com) allows you to operate a similar authentication infrastructure as Google.
What does that mean?
- Single sign-on (SSO) across web and mobile applications
- Ability to support a variety of strong authentication mechanisms (a.k.a 2FA), like U2F security keys and OTP mobile apps, in one place for many apps
If all apps and services were to align with OpenID Connect, we would have a truly scalable and interoperable identity layer for the Internet.