While technically not strictly necessary, it adds more robust authentication methods, and makes it easier to build out other apps if you want to in the future without having to re-do the sign-in process for all of your users. You can have things like 2fa and other things that make it harder for bots to get in and easier for users to stay in. It also makes it easier to keep track of login attempts and notice compromised accounts.

Edit: There are also alternatives like authelia that may be easier to implement. I don’t really trust most web apps to be ultra secure with internet-facing sign-in pages so it just feels like “good practice” to hide behind an auth service whose sole purpose is to be written and built securely. Plus once you learn how to set up fail2ban with an auth service, there will be no need to re-learn or re-implement it if you add a 2nd app/service. Very modular and makes testing and adding new things much easier.

Another benefit is that it has a nice GUI. I can look at logins, add services, stuff like that without touching config files which will be nice for those who don’t like wading through text files to change config.