Why use a multilayered architecture ?Flexibility
It's easier to understand, create, correct, upgrade a very specialized software component instead of a many-specialized one.
Logging and Instrumentation
Rich Domain vs Anaemic Domain
Domain Objects vs Persistence Model (ORM Entities)
ORM ids might differ the DO's identifier (if any).
In many-to-many relationships you might have methods to add an element to a collection and also to set the element's parent this object. This kind of ORM method might not exists/be necessary to DO (domain object).
ORM might need Formula properties but DO not.
ORM might need usage of @EntityListeners but DO not.