OUTPUT clause can return in parallel way both INSERTED and DELETED rows.
It is useful to logging in archive original and new values.
OUTPUT DELETED.value [oldValue], INSERTED.value [newValue]
SQL Server Tips · · · ·
You can read more here, http://beyondrelational.com/blogs/paresh/archive/2011/06/20/output-clause-with-insert-update-and-delete-statements-in-sql-server-2005.aspx
You can read about the I/O, performance improvements offered by the OUTPUT clause and also about it's limitations from my post: http://beyondrelational.com/blogs/nakul/archive/2011/03/21/output-clause-underappreciated-features-of-microsoft-sql-server.aspx