Audio tone control circuits are mainly used for 2 reasons. The first reason is to control the BANDWIDTH of the signal, which is entering into the audio POWER amplifier. It may not possible to recollect the original signal at the speaker if the bandwidth is not limited. The second reason is to satisfy with the music. Tone controlling is nothing but controlling the frequency of the signal to be amplified by the audio power amplifier. Normal audio signal consists of mixed frequency. Low frequencies in audio signals are called as BASS and higher frequencies are named as TREBLE. If you think about separating the frequencies in audio signal then Audio tone control circuit comes on to the picture.