It's a power cable adapter.
Great price. Eliminates lots of clutter when using SATA drive with older power supply. With both plugs combined it stays connected to drive better as well.
Good approach to convert older PC power supplies to power SATA HDD. Better fit using this than seperate power and data cables.
I have been dealing with MCM for over 30 years now. If I didn't like the service I'd would have found a different supplier 30 years ago. For most of my needs they are the 1st stop on my shopping list.