Mixer needs
The signal sent to the console needs to be a strong clear signal. This is why vocalists need to put their mic to their mouth. It’s also why a guitarist needs to turn up the volume on their guitar. Otherwise noise can be heard.
Noise can be picked up within the signal path either via interferences on instrument cables or any time the signal goes through a connection. Noise also comes from microphones as even air particles hitting the diaphragm will produce a sound. Turn on the audio system and open a mic channel. Hear the small noise hum.
The mixer needs a strong CLEAR signal for optimal mixing. This happens with a high signal-to-noise (S/N) ratio which means the true source (guitar, vocal, etc.) is so strong in the audio signal that natural noise is overpowered.
One more point on the S/N. Equipment can be set up so the signal from the stage first goes into a rack component or other off-board processor and that equipment has gain control. If there’s an initial low S/N, by the time the signal reaches the mixer, that noise can sound substantial in relationship to the desired sound. This is another reason you should get a high S/N as soon as possible.
Ideally, you want strong signals with a high signal-to-noise ratio where the signals are all in close relativity to each other. This enables precise volume mixing on the faders.