[Thetawaves]

This improves the impedance of high speed signal transmission - at high speeds the currents want to flow through the ground path as close to the signal path as is possible. Said another way - designers work to reduce the total area inside the electric circuit to support high switching speeds. The additional power pins in high pin count chips serve to reduce the distance between high speed signal paths and their return ground paths. This is actually true for both ground and vcc, but is easier to visualize when working with signals and their grounds.