Any electric device is characterised by power that depends normally on impedance value. We can see a distinct impedance of audio loudspeakers. Here we can count for a definite volume to expect. Unlike loudspeakers headphones have several notions of power for each type of impedance (8-600 Ohm).
And the producers do not state power for each for impedance rates. It means that the easiest way to adjust the headphones amp is to regulate the rate of power that corresponds to the impedance. The higher impedance, the lower is the power, and the lower impedance is the higher power is required. This explains why high impedance headphones need low power electricity and vice versa.
Dependance of impedance and sound quality
Noting the rate of maximum power on the output of the headphones producers try to reduce the risk of its low quality sound. Thus, if you switch maximum volume the amp may lack impedance and power that will cause distortion and unpleasant reproduction of audio signal.
This way the right amp is able to adjust the power and impedance of the audio source and headphones to make them reproduce audio signals with maximum volume and minimum distortion.
Headphones amp and volume
In other words, your phone audio signal may be much louder with a higher impedance of headphones, if you use an amp. This rule works if you use a low impedance output and headphones with a higher one. In this case the sound quality will be low at high volumes. An amp will do two things while adjusting the voltage and impedance of the devices: make its sound clearer and stronger at high volumes, increasing the volume as well.