Why should be the output impedance of an amplifier should be kept small....is there any cause rather than power loss?

 

The output of an amplifier will be supplying current to the next stage, whatever that may be. The output voltage will have been carefully prepared by the amplifier, to some known value, and you don't want the next stage to load the amplifier so heavily that it modifies that nice clean carefully constructed output voltage.

Therefore, the amplifier's output impedance should be small enough to supply whatever current the next stage requires, while maintaining the voltage at the intended value.

The lower the output impedance of an amplifier, the more it is able to provide the intended output voltage under load.

 

An audio amplifier has an output impedance of 0.04 ohms or less at low frequencies. The very low impedance damps speaker resonances.