Sound comes from speakers.
Sound travels with varying directivity and spl and frequency change over distance
Microphones hear sound.
Ears hear sound.
You cannot chose what sounds do and don't enter a microphone at a given frequency range, SPL and direction
You cannot chose what sounds do and don't enter an ear at a given frequency range, SPL and direction
Your onstage amps, if loud enough, give one part of the audience one type of sound and another part of the audience a drastically different sound or perhaps no sound from them. It is optimal therefore to have none of that particular speaker's sound reaching the audience at all
Your onstage amps, if loud enough, leak into any open mic on that stage causing all sorts of havok, phase problems and even destroy your own guitar sound. It is optimal therefore to have none of that particular speaker's sound reaching any open mics (aside from your amp mic if you are donosaur enough for one) at all
Your onstage level from your guitar amp now becomes the minimum amount subtracted from your vocal mics possible gain before feedback, which was already far too low in the first place if the singer is on wedges and not IEMs
Every open mic may need to be filtered or treated differently (and definitely responds to dynamics processors differently) than had that sound not been leaking into them.
Every bit of onstage sound matters to what the audience hears and the most ironic part is how much better a guitar can sound to the audience if it doesn't have to fight all the problems a loud speaker is making onstage.
Also it means nearly every audience member gets a roughly similar experience
Just as a very very very basic scratching the surface