The PSA0017C addresses the problem of attempting to use an aviation headset to listen to a portable music device while connected to the aircraft intercom. The 600 ohm aviation headset requires more power to drive than low-power portable music devices are capable of delivering. Furthermore, music at levels high enough to overcome the high-noise environments of the general aviation cockpit can easily drown out other intercom audio.
This is the feature that the manufacturer forgot. The adapter replaces the manufacturer's cover which is used to cover the 5-pin plug on the unused (non-microphone) earcup.