The reason for all these modes is that they depend on having sensors working. For instance ideally you have the angle-of-attack vanes working and airspeed sensors working and with that you can guarantee that the plane is not going to stall.
If you don't have those sensors the control system can offer less certainty and less amenity.
A similar situation exists with the engine control unit in an automobile which uses the oxygen sensor to know it is injecting just the right amount of fuel into the cylinders but won't leave you stranded if the sensor fails.
