Here's a list of ten reasons why you should consider an ORM tool. Now not all ORMs are created equal, but these are key features that a first class ORM will handle for you.
1. Facilitates implementing the Domain Model pattern (Thanks Udi). This one reason supercedes all others. In short using this pattern means that you model entities based on real business concepts rather than based on your database structure. ORM tools provide this functionality through mapping between the logical business model and the physical storage model.
2. Huge reduction in code. ORM tools provide a host of services thereby allowing developers to focus on the business logic of the application rather than repetitive CRUD (Create Read Update Delete) logic.
3. Changes to the object model are made in one place. One you update your object definitions, the ORM will automatically use the updated structure for retrievals and updates. There are no SQL Update, Delete and Insert statements strewn throughout different layers of the application that need modification.
4. Rich query capability. ORM tools provide an object oriented query language. This allows application developers to focus on the object model and not to have to be concerned with the database structure or SQL semantics. The ORM tool itself will translate the query language into the appropriate syntax for the database.
5. Navigation. You can navigate object relationships transparently. Related objects are automatically loaded as needed. For example if you load a PO and you want to access it's Customer, you can simply access PO.Customer and the ORM will take care of loading the data for you without any effort on your part.
6. Data loads are completely configurable allowing you to load the data appropriate for each scenario. For example in one scenario you might want to load a list of POs without any of it's child / related objects, while in other scenarious you can specify to load a PO, with all it's child LineItems, etc.
7. Concurrency support. Support for multiple users updating the same data simultaneously.
8. Cache managment. Entities are cached in memory thereby reducing load on the database.
9. Transaction management and Isolation. All object changes occur scoped to a transaction. The entire transaction can either be committed or rolled back. Multiple transactions can be active in memory in the same time, and each transactions changes are isolated form on another.
10. Key Management. Identifiers and surrogate keys are automatically propogated and managed.
Please feel free to add your additional via the comments....