I mean, it has the same benefit of other SaaS, you get to avoid building something and can spend that dev time on building something that solves a unique proble

I mean, it has the same benefit of other SaaS, you get to avoid building something and can spend that dev time on building something that solves a unique problem AND you have the benefit of knowing that you get to focus 100% on your app or site's problems and features, and that you have the entirety of Auth0 focusing on keeping your authentication working. I can promise Auth0 is better at building scalable, secure, and resilient authentication solutions than most dev teams, and I've been on a team that's built out a 1000s of logins/hour and 100k requests/hour enterprise grand IDAM solution. If it's data security or something else that's your concern, you can host the data in your own database with their enterprise package. General disclaimer: I'm a paying Auth0 customer but just use it for authentication, and it saved me a hundred hours of work for a pretty reasonable price.