This hard drive cable carries the data and power to the 2.5" SATA hard drive. Two in one! Well, I guess technically it's two in two; the power and data cables are separate but wrapped together, and generally referred to as a single cable.
923-00565 Hard Drive Power/Data Cable for iMac 21.5-inch Late 2015 A1418 MK142LL/A, MK442LL/A MK452LL/A This is a replacement hard drive data and power cable for the Late 2015 iMac 21.5" models.