When you turn off your car Pump the Brake Pedal. The first press is fine, each additional press has less boost.
At road speeds the steering does not need much boost. Many cars cut the boost at high speed so the steering is not so sensitive.

The old linkage an a pot on the "throttle butterfly" system would choke the engine of air is there was an electronic failure of the system. The "by wire system" needs some sort of redundancy.