Boeing has introduced this for most if not all of their types. I'll simply let Boeing answer this for you:
These pitch and power settings provide a simple reference setting for the crew to use for a short period of time while the initial steps of the checklist are accomplished. These settings do not ensure a level flight or constant airspeed at any particular altitude/airspeed/weight combination. However, they do ensure that at any and all weight/altitude combinations, the aircraft will accelerate from low speeds, and slow from a high speeds, as we cannot assume the aircraft is in stable flight when the NNC is run. It is more likely that the crew or the autopilot has destabilized the airplane as a result of erroneous airspeed indications prior to identification of the need to run the NNC. These memory item pitch and power settings often result in a significant climb (from lower altitudes) and can also result in a gradual descent (from higher altitudes).
Remember unreliable airspeed is rarely obvious as in your example. Things don't go haywire it can often be subtle to begin with. Try blocked Pitots on takeoff in the sim and see how many crews lose control of the aeroplane before they have a clue whats happened. Boeing assumes you may lose control before any confirmed diagnosis hence this procedure to keep you alive along enough to try and work it out.