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”