Two different potential voltages need to be common grounded so there is no floating ground to create a static charge and burn out you're electronic components with spurious spike's. Two different voltages like 177v & 16.7volt, are grounded in this drawing & you will always see this connection in a air conditioning system where motor components are used & needed.
The motors produced a large back EMF which in turn is a static charge on the field windings, this charge passed back to ground normally but can damage electronic components if grounding is improper or the field windings start to fail.
All components hooked to mains must be grounded by electrical code. So there is never a chance of floating ground potential.
