The operation of rolling back all uncompleted transactions and rolling forward all completed transactions is called recovery and is required to restore the integrity of the database. Restoring a database backup or applying a transaction log backup recovers the database automatically at the end of the restore operation. Additionally, recovery occurs in a database each time the database is opened, for example when Microsoft® SQL Server™ starts.
You may need to recover a database manually when:
To bring the standby online, recover the database.
To recover a database without restoring