First, persistency. You data lifecycle may not be directly proportional to your applications lifecycle. You may need it even after the app is shut down.
Second, RDBM systems provide a well defined memory/storage structure and API - "structured query language". This enables you to easily query your data and acquire suitable views that are beneficial for your applications purposes.
Third, It's always better to outsource your data layer to a battle tested, and trustworty database then trying to reinvent the wheel.
So this paves a road for you to focus on your business logic than splitting that focus for the data layer and business logic.