That’s my point: IdentityServer (for example) is what enables any org capable of running a website themselves to integrate with other OIDC-conformant identity solutions via federation without having to cede any control over their identity system.
A good business case is a where a company (or public department) outsources their identity system to a company that doesn't have a 24/7 emergency phone line and people can't login and it's a business emergency - but I recognize this scenario is the same as "outsourcing is cheaper than in-sourcing, but outsourcing with the same level of quality and service as in-sourcing costs more than in-sourcing". YMMV.
A good business case is a where a company (or public department) outsources their identity system to a company that doesn't have a 24/7 emergency phone line and people can't login and it's a business emergency - but I recognize this scenario is the same as "outsourcing is cheaper than in-sourcing, but outsourcing with the same level of quality and service as in-sourcing costs more than in-sourcing". YMMV.