Re: op-amp
Any of amplifiers, not only opamp, is expected to be ideal, means that it has infinte input impedance, and zero output impedance.
Infinite input impedance means that the amplifier does not draw current (load) the previous stage. All the previous stage output voltage apply to the amplifier input without current, so the previous stage does not consume additional power from power supply (higher efficiency), and the amplifier does not make input signal distorted.
In the same way, the amplifier with zero output impedance will load all output voltage to the next stage and the output signal will not be distorted.