disconnect or kill session

hi,

I try to find an interest to use "alter system disconnect session immediate" rather "alter system kill session". The msg in output is not the same (ORA-).

The online doc explains, it's the same. But why create the same action with two different statement ? 

SQL> show user
USER est "SYSTEM"
SQL> select * from v$mystat;
select * from v$mystat
                     *
ERREUR Ó la ligne 1 :
ORA-03113: fin de fichier sur canal de communication
ID de processus : 0
ID de session : 146,  NumÚro de sÚrie : 437

alter system kill session '146,439';

SQL> show user
USER est "SYSTEM"
SQL> select * from v$mystat;
select * from v$mystat