Why put a DAO layer over a persistence layer (like JDO or Hibernate)

Question

Data Access Objects (DAOs) are a common design pattern, and recommended by Sun. But the earliest examples of Java DAOs interacted directly with relational databases -- they were

Answer 1

The purpose of all this introduction to layers was to make maintainability easy and simple.

1. Data Access Layer
2. Business Layer
3. Presentation Layer

The purpose of the 1st Layer (Data Access Layer) is to deal with the database logic and prevent the Business Layer from knowing any of the DB details.
The Data Access Layer uses POJO or EJBs (DAO) to implement IoC and POJOEJBs uses Hibernate or ORM mapping to actually deal with the Database Layer.
So, if you want your business logic should not care about which, what & how a database is being used, accessed and updated and you want DAO to take care of this
DAO can support the logic of changing different tables to support operation by making a number of hibernate calls.
In essence, you are implementing a layered approach in Data Access Layer by breaking its functionality again in two layers aka DAO and Hibernate.