Log backups are an important part of a full database backup. SQL Server backs up the transaction log as part of a full backup to make the full backup transactionally consistent when the full backup is restored. Before you can back it up, the transaction log needs to be accessible and complete: SQL Server must have access to all log activity that has occurred throughout the full backup, and the log can't have been cleared. However, the default transaction log backup behavior is to back up and then clear what was backed up. If log backups were allowed during a full backup, the full backup couldn't back up the log and thus would be transactionally consistent during a restore.

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