Good question, I don't know Bosch's reasoning but my guess is primarily two fold:

1) Achieve various pressures based on load type- ie pots and pans get strong pressure, everyday cycles medium pressure, china cycles deliver low pressure. GE for example would either introduce air into the sump boot or do a reduced fill to soften the water on the china crystal cycle.

2) Compensate for the tall tub low water design-

a) To prevent drawing in air or cavitation, the pump slows down as water is returning into the sump then speeds up to deliver it to the wash arm. As soon as the pump starts to draw in air, it slows, waits for the water to return, then ramps up. Its a way of using less water with the same sump, pump and spray arms.

b) To purge trapped air bubbles out of the pump and sump.

c) To warm the air inside once the door has been opened or the machine has drained and re-filled. The rapidly expanding cold air can force water out of the bottom vents where the door linear meets the tub floor- so by warming the air inside through brief bursts of water there is less cold air to expand at start of wash. 

I'm sire there are other reasons, but these 4 come to mind.

Not sure- maybe energy. It takes less pressure to rinse dishes than to clean them, in theory, so running the pump at low speed saves energy without sacrificing performance. 

Me personally I like across the line motors. VFDs add complexity and various failure points.  Electrolytic caps have a finite lifespan, so if those are present in a machine its a given expiration date.

A pump running at lower speed when driven by a typical VFD will definitely consume less electricity. If its a 3 phase motor, as are most VFD motors, current equally goes up or down on all 3 phases. 

If the machine works for you, if it makes you happy, then I am happy. If the Bosch works for you then it works for you, thats all that matters. 3 phase induction motors are the best out there. They have the lowest vibration, smoothest power output, great torque and are among the most efficient. I don't know what the slip on that motor is, but if driven by a high frequency that motor may not even get super hot to the touch making is bar none while extending its life. 

Thanks for bring those threads to my attention, I will check them out! :) 

Ah, ok, that makes sense. I was thinking a dry rotor induction pump. My mistake in assuming.  

Can be produced from a single phase service via a variable frequency drive. AC power comes in, is rectified to DC, smoothed, then converted to 3 phase out-power via semiconductor switching. 

Jerome, look into thermal shock. After a main wash, the dishware is very hot, perhaps say 150*F. When water enters the tub for rinse, particularly with long runs of water lines during the winter coupled with a water heater set to less than 120*F, the water will be cold then change to warm as its filling. It will not be 150*F. As such you've got 150*F dishware being hit with say 90*F water. To reduce stress the water is gently pulsed, gradually cooling the dishes down over a protracted period of time. 

Think take glass out of an oven, then pouring ice cold water on it. Often times it will crack or shatter. Same thing, just more extreme.