It enables that faders output on the given mix. On non LR mixes there is also an Pre/Post fade button adjacent.
One example of usage might be for a bass guitar, I want to put the signal back to the bass amp, but don’t need it in the FOH setup, since the speakers can’t deal with those frequencies. I can put the Bass amp on Mix4, with a post fade send from the Bass channel.
Then I take the Bass channel out of the main mix – but the fader still does the right thing, the volume is controlled normally, but the sound comes from the Bass amp, not the FOH speakers.
It’s therefore much less “aggressive” than a mute button.
Does that help?
(Note that the mixer has this option when you go to a mix on the right section, then hold down PFL or Assign and tap the select buttons for each channel)