Difference between migrateuser and adduser manually

Just want to clarify what will be the difference between the two:

1. I migrate a user 'xyz' from one AD domain e.g, DomainA\xyz to another AD Domain, DomainB\xyz using stsadm -o migrateuser cmdlet


2. if i just manually add 'xyz' user to new AD Domain, DomainB\xyz and the same xyz user also exist in old AD Domain, DomainA\xyz

whats will be the difference with user 'xyz' added manually and migrated through stsadm cmdlet?