I think need is always contentious. You don't need oidc there are definitely alternatives. Using oidc for first party login makes sense to me mostly because of standardization - and allowing the client to fetch their own credentials. I’d actually say the question is if you need scopes and for most products the answer is no

But yeah for a typical api driven system with a mobile app I think registering clients, letting them get their own credentials, using JWTs etc the rest of the stack makes a lot of sense. I think/worry that for most companies you’d end up creating a worse version of oidc / hydra pretty quickly as feature requests start coming in for things that are common in most apps