Using a database (whether it’s a traditional SQL database or an OTP database such as Mnesia) makes sense when you want to persist data beyond application restarts. This might be true for parts of your data or all of your data, depending on your application.
I personally tend to just use an SQL database right away because it usually fits my application’s need anyways.
But if that’s not the case, you can always add a persistence layer to your stateful application that takes care of storing data when required and loading it when it gets restarted. With this approach you can leave most of your code unchanged and simply add code for storing/retrieving in/from the DB to your own state management functions.
