 The shift towards multi-rail power supplies can be seen as when new tech such as the Pentium4 or Athlon64 platforms, along with the introduction of GPU's which required more power than the AGP/PCIe slot could provide, more and more power was needed for that progression.When given a dedicated +12V rail for exclusive use by the GPU, and another dedicated rail for the rest of the system, the increased power demand was met while maintaining a simple design, ideal for (continued) mass production and cost control for a more complex unit.The later introduction of high power-demanding multi-core and multi-gpu systems, saw a split from two to three or four rails.