The so-called PA is understood differently from different professional levels, Professional Audio or Public Adress, that is, the audio used professionally and broadcasting used in the public.
The basic requirement of PA
is to convey information to the audience with high-efficiency audio. All it cares about is that the voice is transmitted and accepted by the audience. There is no special requirement for good or bad listening. To have a good sound, there are high-quality and efficient speakers with sub-channel, so that the sound of the concert can be more clear and undistorted to the audience's ears.
Then there is the birth of the monitoring system, in the past there was no monitoring system, so it is difficult for musicians to hear their own voice, and the emergence of the monitoring system is equivalent to giving the musicians performing on the stage another pair of ears, so that they can clearly hear their own voice.
Finally, there was no SR in the early stage, so the voice controllers at the concert did something more like PA (Public Adress) than SR, because they wanted to convey good sounds to the audience, rather than simply play them out loud. However, the P.A. performers are insensitive to these voices which do not exist from beginning to end, and may also greatly affect the overall performance level.