The input and output resistance of an amplifier, should be a good match for the signal source and the signal destination. e.g. if you are using a crystal microphone into an amplifier that is driving a loudspeaker. the crystal microphone can deliver a good voltage (.2V) but only at a very low current (.2 micro amps), if your amplifier has an input impedance of IK ohms, then it will get the .2 micro amps, which will correspond to an input voltage of . 2 X 10^-6 X 1 X 10^3 = .2mV. if the amplifier had an input impedance of 10 M ohms then the input signal would be (10/10+1) X .2V ~ .18V. So having an amplifier with a high impedance input is of great value if the signal source is high impedance.
Frank