User authentication allows you to create a login module that you can make mandatory or not upon entering your app.
The ability to close part or all of your app is a key feature for many app creators. It will allow you to offer content and features exclusively to members of your community. In addition to protecting your content, it will allow you to encourage your users to create their own profile, and will therefore allow you to better know your audience.
To show you a concrete example of use, here is the login page and the user profiles proposed in the Dali Corp app which is an internal app for companies. Intended for Dali Corp employees, the app is closed to users who don’t have a login.