right, what web and vicious said. the stock brakelines expand/bulge when you step on the brakes due to the pressure buildup. this makes the pedal feel "mushy", it goes further down and is less responsive when you step on it. also, the brakes engage/clamp at a later point in the brake pedal travel.
the ss braided lines prevent the lines from expanding/bulging, especially when they get hot, and provide a more positive brake feel. you don't have to push the pedal down as far to activate the brakes. the pedal holds at a higher point in the travel.
while they don't increase braking force or brake output, or reduce braking distance, they do reduce DRiver braking input force, which does improve daily driving, as well as on the track.
for 80 bucks, a pretty nice simple bpu.