Basically Entity Beans are used to connect to the Database (it is not a good practice to connect to a database thru a Statless or Stateful Session bean) thru a JNDI Name.... JNDI Naming makes it easier to connect in an enterprise becuase it abstracts the datasource classname etc from the developer,
The main advantage of using EJBs over Simple beans is that J2EE has a moto of develop once deploy many, if there is a slight change in business logic etc, the developer can change the business requirements and redeploy the bean and the client wouldnt have a whiff about it.
Other than that there are many features in EJBs, for example the Container Managed Persistance in Entity Beans enable you to find, create, delete, update records in your database without going thru the hinderance of writing SQL Scripts which increases database integrity, another feature of Entity Beans is Container Managed Relationships which maintains schema integrity....
These are just some benefits that I guess are benefitial in a large environment over legacy systems when we compare EJBs and normal Java classes.