One of the security design principles is scope reduction and data minimization. By storing customer personal data in the Databunker, you basically minimize the attack outcome from your existing database, thus minimizing the business risk factors.
Suppose, the bad actor finds an SQL injection in your web app database. He might get access to some data. This data will not be personal as this information is stored outside of your existing database (in Databunker).
When you clean up your databases from personal records (PII) and use the Datbunker token instead, you are basically doing data minimization and GDPR scope reduction.