when to use Serializable backing bean in JSF?

when to use Serializable backing bean in JSF? is there any factors to go for serialization like View Scop开发者_开发技巧e, Session Scope, Application Scope or is anything else which requires bean to be serialized.

I'd say that session-scoped beans are eligible for serialization, because the the servlet container tries to serialize everything in the session in case it (the container) is restarted.