The primary use of a speed-holding mode would be in the climb - holding a specific speed (which varies by aircraft and by the aircraft weight) will give you specific climb performance. Either best rate (shortest time to your altitude) or best angle (useful when necessary to clear an obstacle, but not often a requirement in this game).
For most propeller driven aircraft, the two airspeeds are very similar. In jets, the airspeed for best rate of climb is significantly higher than best angle.
Mike
