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.
923-01672 Hard Drive Power and Data Cable for iMac 21.5-inch Mid 2017 A1418 MNDY2LL/A, MNE02LL/A * EEE CODE :, , N/A, * Model Compatible Number :, , A1418, * EMC Number :, , EMC 3069, * Part Number for Manufacture :, , N/A, * Part Name :, , Hard Drive Power and Data Cable, * Manufacture Model Year :, , Mid 2017, * Mac Computer :, , iMac 21.5-inch 3.0/3.4 GHz Core i5 (i5-7400, i5-7500), , iMac 21.5-inch 3.6 GHz Core i7 (i7-7700)