Instead of using Unity Catalog or the built-in Databricks Hive metastore, you have the option to bring your own existing metastore to Databricks. Databricks clusters can connect to existing external Apache Hive metastores or the AWS Glue Data Catalog.
Using external metastores is a legacy data governance model. Databricks recommends that you upgrade to Unity Catalog. Unity Catalog simplifies security and governance of your data by providing a central place to administer and audit data access across multiple workspaces in your account. See What is Unity Catalog?.