When you're just starting out with MongoDB e.g. playing with it on your home PC, you don't need to worry about setting up database users etc, because authentication is disabled by default.
But once you start thinking about building an application with MongoDB, you'll probably need to think about which users should be able to read/write to which databases/collections. For example, ordinary users should probably only be able to read/write certain collections in a given database, while your application admin would need to be able to do things like create collections and create indexes etc in the application database.
It's just a learning example, so don't rely on this for your production systems - always rely on the MongoDB documentation for real-world applications! But it might help you to get started with MongoDB's approach to user permissions etc.