Beginner java question: Serializable

Can somebody tell me under what condition should I consider implementing 
Serializable interface?  I've a book that shows how to use it to write an 
object to a file and retrieve it back.  I think I understand what it does but 
I'm not sure when should I consider implement it.  
 
(Note:  Last night when I was trying to deploy a web app to Tomcat, it failed 
and the log said that one of the bean is not serializable.  After some digging 
I finally found out that I'm missing <%@ taglib uri="struts/logic-el" 
prefix="logic" %> in my jsp.  No sure why Tomcat is giving me the not 
serializable error, which lead to my question).