Answer:
Digital signal processing is used instead of complicated analogue circuits.
It is used in mobile phones and PA amplifiers to prevent feed back (howl round).
A microphone placed near a speaker, will cause the microphone to hear itself and cause an annoying howl. By applying some of the mic signal in antiphase to the local speaker, this can be stopped. This allows simultaneous conversation using hands free, which would be impossible without digital signal processing.
It can also be used to clean up old recordings to remove noise.
All sorts of effects can be applied to an audio signal, such as reverberation, echo and tone.
Once the signal is in digital form, all the processing can be done by a program using mathematical processes and applied digitally to the original signal. Changes can be made by simply altering a few lines of code.
before digital processes, sound had to be altered by complicated, expensive, circuits that could not be changed easily.