Cat II/III use ILS which send out signals on the runway(s) in question. IIRC, those signals need to be monitored to make sure they're with-in certain tolerances because II/III is used in minimal visibility conditions, so there's very little time to correct things at the last minute. It's about trusting a whole bunch of equipment to work properly because you're basically flying with a blind fold on otherwise.

This Garmin G3000-based system primary uses GPS/WAAS (and perhaps VORs) to get location data, so accuracy may not be with-in necessary tolerances for day-to-day use, but it's better than nothing for emergencies.