The basic benefit of using ORM tools is to facilitate the principal of separating the business logic from the data access in a multi-layered application. If you can successfully build a data access layer that is responsive to changes in the database (by changing your mapping), there is less overall code for you to have to muck around with when changes are made.
Separation of the layers is typically the goal of 3 tier or n-tiered applications, and ORM is a good method of doing that.
