Use the Object/Relational Map pattern when:

You want to hide the physical data model and data access complexity from application logic and domain objects. Doing so keeps these components cleaner and more focused on the business objects and processes that they model.
You want to encapsulate domain object mapping within a single component so that you can adapt to data model changes without changing application code or domain object definitions.
You want the versatility to map domain objects to multiple data models without changing application code or domain object definitions. This versatility lets you integrate your application with multiple data models, regardless of how they are defined.