Usually the extra 12V lines are for providing power to additional video cards via the PCIe bus. Not all cards consume a lot of power through there. RX480 being the most recent culprit of even exceeding board specs. If you had, say, three of those, it would be a struggle for a single 8-pin connector to power all the cards and the CPU. Thus the additional wires.
Your power supply doesn't provide any more or less power, it just prevents excess heating, and thus voltage drop, on a smaller number of wires. (And 12V PC wires are very underrated, if you go by the rated specs of twisted strand copper they can handle a lot more than what the 'standards' say)