Often when people ask for Oauth they really mean OAuth2/OIDC, and what they really want is managed authentication, with further authorisation being managed at the application level rather than through oauth.
The advantages are there:
1. Reducing sign-up friction. Despite how much this crowd dislike such things, it's much easier for most people to hit "Use google login" or "use facebook login" than create a new account. One less password to remember ( or one less place to store the same password you use everywhere ) for people without password managers.
2. People trust "Google" or "Facebook" more than they trust random service they're just checking out. Having that "sign in with google" extends that trust to your service.
3. If you ever do need Authorization, it's much easier retro-fitting that if you already have OAUTH than trying to retro-actively fit it all to get that authorization.
4. Letting someone else take care of user account cancellation. Businesses hate it when there's no OAUTH, because they don't want to have to track down and change / cancel accounts when someone leaves their organization. They want to suspend/remove them from their Azure AD / Octa / etc and that user instantly becomes persona-non-grata across all services. They might also want to have different groups at their end, so they're still the same identity, but lost authorisation to log into <Service> now they moved from an ops role into development, for instance. They're still the same identity and same authenication in this case, but should no longer be authorised for your service.