just a short understanding-question:

User user =new User();
user = em.merge(user);
em.persist(user);
System.out.println(user.getId());

why do I have to use this "em.merge(user);" to create this Entity? If I leave this line out I get lots of exceptions:

16:54:15,500 INFO [STDOUT] javax.ejb.EJBException: null; CausedByException is:
 detached entity passed to persist: com.example.entity.User
16:54:15,501 INFO [STDOUT] at org.jboss.ejb3.tx.Ejb3TxPolicy.handleExceptionInOurTx(Ejb3TxPolicy.java:46)
16:54:15,501 INFO [STDOUT] at org.jboss.aspects.tx.TxPolicy.invokeInOurTx(TxPolicy.java:70)
16:54:15,502 INFO [STDOUT] at org.jboss.aspects.tx.TxInterceptor$Required.invoke(TxInterceptor.java:134)
.....
....

I simply don't understand, why I have to use it, cause in all the examples it isn't used at all ...