The Primary HA Screen manages Secondary HA Screens in an HA cluster. A passive HA Screen within a HA cluster mirrors the state of the active Screen, which can be the Primary or a Secondary HA Screen. When the active Screen fails, the passive Screen that has been running the longest takes over as the active Screen. Primary means the system is the HA administration host for the HA configuration. It does not mean that it is the active host, necessarily.
You must use the unique HA interface address for administration. If one of the shared addresses is used, then that address will always resolve to the HA Screen that is currently active. Since the active host is not necessarily the Primary administration host, there is no other way to ensure that you are communicating with the correct host.
If you do not do this, then, if the remotely administered Primary HA Screen is shut down, the connection will be lost and the administration GUI will hang immediately. You can still administer the active HA Screen from the command line, using the command ssadm, but you will be unaware that you are administering a Secondary HA Screen that will not propagate the configuration to any other HA Screen. The configuration gets overwritten when the Primary HA Screen is up again.