When and why JPA entities should implement the Serializable interface?

According to JPA Spec:

If an entity instance is to be passed by value as a detached object (e.g., through a remote interface), the entity class must implement the Serializable interface.

"JSR 220: Enterprise JavaBeansTM,Version 3.0 Java Persistence API Version 3.0, Final Release May 2, 2006"

This usually happens if you mix HQL and native SQL queries. In HQL, Hibernate maps the types you pass in to whatever the DB understands. When you run native SQL, then you must do the mapping yourself. If you don't, then the default mapping is to serialize the parameter and send it to the database (in the hope that it does understand it).

You need your entities to be Serializable if you need to transfer them over-the-wire (serialize them to some other representation), store them in http session (which is in turn serialized to hard disk by the servlet container), etc.

Just for the sake of persistence, Serializable is not needed, at least with Hibernate. But it is a best practice to make them Serializable.