The adapter cable converts between TTL signal levels (AP side) and RS232 signals (console side) and draws power from the AP.
While I agree the (list) price is high, it's definitely worthwhile to order one of these when deploying any of the APs that have this interface (RAP-3, RAP-155, AP-103, AP-103H, and more to come).
/Onno Harms
#RAP155