I am seeking a clarification on the Persistent Volume in Kubernetes.

In this below example, the /my-test-project is in the persistent volume. Then, why I need these mounts as technically my entire directory /my-test-project is persisted? How would these mountpath and subpath help if entire directory is persisted?

volumeMounts:
    - name: empty-dir-volume
      mountPath: /my-test-project/data-cache
      subPath: data-cache
    - name: empty-dir-volume
      mountPath:  /my-test-project/user-cache
      subPath: user-cache
  
  volumes:
  - name: empty-dir-volume
    emptyDir: {}