Torque boxes tie the subframes to the rocker boxes and add to the overall torsional rigidity of the chassis. Probably makes the greatest difference between opposite corners (diagonally accross the chassis). When used in conjunction with frame connectors stiffening the floorpan area, you're essentially using the frame connectors and rocker boxes to build horizontal trusses through the floor area at the sides of the chassis and greatly improve the entire floor area's resistance to deforming under load.