A brief explanation why…
When attached to a motherboard's PCIe x16 slot, a card such as the GTX 1080 can draw as much 75W on the +3.3V and +12V rails (3.3V x 3 A + 12V × 5.5 A). This is true for specifications up to and including PCI Express 4.0.
Plugging a 6-pin PCIe power connector into said graphics card provides up to another 75W and an 8-pin connector offers an additional 150W.
PCIe x4 and x8 cards can pull as much as 25W from the motherboard's slot while x1 cards are limited to 10W unless configured as a 'full-height', 'high-power card' to tap the aforementioned 25W.