The Mystery of the XE2 SFF PSU

One of the upgrades I performed on my 3020 SFF was the VX372 power supply from the XE2 SFF variant of these machines. 315W @ 4.4A, compared to 255W @ 4.6A for stock. Fit perfectly, and runs cooler under the same load. Mission accomplished. There's been much talk here about the comparatively low output of the stock  PSU, but the factory option Radeon R7 250, pulling 75W, ran very comfortably. And the newer cards that more than double performance (Radeon Pro WX 2100, and all the GeForce GT1030's out there) are doing so at half the wattage required. 

There remains a mystery with this power supply- an additional six wire connector. ALL the low profile graphics cards I've seen are rail powered and don't need additional power. So what is this for? On a service call long ago, I remember seeing a *powered* PCIe card residing in the x4 slot, looking much like the image below, connected to that cable. No exterior port that I can remember. *Not* a wireless or USB card. Who knows what this is or what it's for? (Dell folk can chime in.)