In that specific picture, I use the bottom plug to connect it to my PSU. But the top plug is unused, what's its purpose?
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a little black duck
Hmm, never come across that before, none of my fans have that arrangement. But I suspect it's because fans draw very little power it acts like a splitter so you can connect other peripherals via the top molex without the need for tying up more of your psu available cables.
those type of fans are used to connect directly power supply.