SSO is really important in the "few tools, many users" case, but just as important in the "many tools, few users" case. I'm self hosting dozens of tools, and without SSO I'd have to set up username, password, TOTP and WebAuthn for each and every one of them, my 2FA app would be 90% my own services.
With SSO though, it's much simpler. I can just run an OIDC server and log into all my self-hosted services once, and I can use all of them. Vaultwarden is an exception to the rule though, as you can't really bootstrap that in the individual case.
Another use case I'm currently exploring is for sharing netflix/prime/disney+ passwords with roommates, partners and friends. They just sign in with their Google/Apple/whatever account and get access to the shared streaming provider passwords.