The 8-pin PCle power cable is designed to supply power to a graphics card that requires more power than the PCle slot can provide. A workstation that is running multiple GPUs in an SLI configuration would need this cable to ensure adequate power delivery and stability. The other cables are not suitable for this purpose, as they are either meant for the motherboard (4+4-pin ATX12V and 8-pin ATX12V) or for older devices (4-pin Molex) https://superuser.com/questions/849265/is-there-a-difference-between-8-pin-eps12v-and-pci-e-connectors