Do I need the `CREATE` directive in `/etc/mdadm/mdadm.conf`?

Yes.
The CREATE directive in mdadm.conf sets system (default) parameters for newly created RAID arrays and corresponding device nodes in the /dev directory.
For example, it specifies:

  • The owner (owner) and the group (group) to which the device will belong.
  • The access mode (mode), in this case 0660 (rw-rw----).
  • Whether to automatically create devices (auto=yes).

Thus, if you do not explicitly pass these parameters in the mdadm --create command when creating a new RAID array, then mdadm will take them from the CREATE line in the configuration file.