When your database is damaged and you are trying to do a last-ditch backup of the log before starting restoring over it, or when for other reasons you're restoring or moving a DB and want to ensure that no transactions are lost.
It's a backup log command. WITH NO_TRUNCATE if the log is damaged, with NO_RECOVERY if it's not.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability