LT1 Clutch Bleed Question

Ok read about everything regarding bleeding the clutch on the LT1 drivetrain. One question.

What does loosening the master cylinder have to do, ie how does it help, with bleeding the system?

I haven't done this step and should try but would like to know what differencve that makes in getting a good bleed.

I never have. I'm not sure why it says that.

Thanks for the reply.