Common Emitter, Common Collector and Common Base amplifiers have their specific purpose with differing voltage/current gains. For example, I know for a common collector has a voltage gain of approximately one, lagging behind 0.7 volts and has a high current gain.
But each amplifier configurations also have differing Rin and Rout characteristics, namely
CE Amplifier - Rin ~ moderate, Rout ~ moderate CC Amplifier - Rin ~ large, Rout ~ small CB Amplifier - Rin ~ small, Rout ~ moderate
Why is this information regarding input and output resistance important in these amplifiers? What purpose do they serve?