When you activate this button, the sound of the desired line is turned off and no sound is played from this line of sounds. Usually in analog mixers you don't need all the lines at once, so try to mute the extra lines and lines of sound.
This way you don't have to worry about configuring the other lines. Also, a line may have a small amount of noise that is not easily audible; Turning off the extra lines in these cases will give you a cleaner output sound.
Also, if the other person is with you when she is working, it will be easier for them to understand what you are doing because she will not be involved in adapting the rest of the sections.
There are many uses for this button, for example, before connecting the microphone to the mixer, there is no need to mute the microphone line output sound, just press this button and then plug in the microphone.
Note that the Mute button circuit is separate from the PFL button. In fact, turning on the Miut button does not silence the signal. So you can activate the mute button and use the PFL button whenever you reach this conclusion, you need this line; Turn off the mute button to play the sound.