jschmid explained pretty much everything.. i'll just give a quick review of stainless steel brake lines.. it reduces fade under several hard braking such as road course racing.. other than that, it adds the benefit of a more direct brake feel.. you actually feel a larger range of modulation. the amount of pedal input will show in the amount of braking the car does..

Quick illustration:
Pedal input in a range of 1-10
1 2 3 4 5 6 7 8 9 10

What the braking system applies on stock lines:
1 2 2 3 3 4 7 7 8 8

with the stainless steel brake lines, you have better control of how much braking is applied and better feel.