From the documentation:
The Backup BizTalk Server Job creates synchronized backups of all BizTalk Server databases by using full database backups and transaction log backups, in conjunction with a type of transaction known as a marked transaction. Marked transactions are transactions that place a mark into the transaction log of all databases participating in the transaction. The marked transaction blocks new distributed transactions from starting, waits for the distributed transactions that are currently running to complete, and then executes to place the mark. For more information about marked transactions in SQL Server, see http://go.microsoft.com/fwlink/?LinkID=137859.
The mark represents a transaction point that is consistent across all databases; you can use the mark with subsequent log backups to restore your databases to that point.
For each BizTalk Server database, the Backup BizTalk Server job creates a marked transaction log backup every time it runs, and it creates a full backup based on a time period that you specify.
Without using this Biztalk backup job you risk having backups that restore from a SQL point of view but are useless to Biztalk because the databases are in an inconsistent state.