If the aircraft is using an Inmarsat SATCOM datalink for transmitting engineering data to company maintenance operations, the IRUs need to be on and aligned. This is necessary in order for the steerable SATCOM antenna to find and lock on to the appropriate geostationary satellite for the aircraft’s current location. GPS coordinates alone are not sufficient- the antenna controller needs to know the aircraft’s current pitch/roll/yaw orientation - (in particular, its heading) - to find the proper satellite. That information can only come from the IRU/ADIRU.