From the GM site:
Hydroforming reduces the stress and stretching of the steel during manufacturing so the frame retains more of its strength. Eight cross members, rather than the traditional four or six, are built into the frame, boosting the structural strength to deliver a better ride quality, handling control, and reduce noise and vibration. GM pioneered automotive hydroforming on the Chevrolet Corvette and used this process for the first time for a truck frame on the Silverado, Tahoe and Suburban. The Chevy TrailBlazer marked the first time hydroforming has been used for the entire length of a truck frame's side rails.