They do not replace anything although the front brace will at least require some bending of sheet metal behind the front bumper... I have these on my Nismo.

The front bar replaces 2 little bars in the front

Nismo chassis brace set review
 

After installing this chassis brace kit and driving the Z for over a month w/it, I don't feel that this kit is worth $600. Maybe $200 on a seller's lucky day.
I also couldn't install the rear lower brace that attaches to the diagonal braces due to interference with my AAM Comp exhaust.
I am willing to sell the rear brace for $100 plus shipping. Please note, its powdercoated black. PM for more details.

In a street application, the benefits are probably margninal at best (you probably will not feel a difference). These braces and most any other brace application, will only reveal their true purpose and benefits in a track setting. Most people install full coilovers and strut tower bars for that "show car" appeal. How these types of components affect the behavior and handling of the car (both positive and negative) will never be revealed on surface streets.

Nice but a pretty penny.