The most serious applications need to persist some amounts of data. This data could be transactional data, user preferences, or the current state of the application. There are many ways that data can be persisted in our applications. Determining the appropriate way to persist this data can be one of the most critical decisions that an architect needs to make.
As an architect, we should separate the actual data storage from the business logic. This will allow the application to change how the data is persisted in the future without having to change the business logic. This is called a data access layer.
It is important when designing an application to design a good data access layer because having a good data access layer will make the code much easier to maintain as the requirements change. If we separate the data access layer from the main business logic, then if the backend data store changes, only the code within the data access layer needs to change.