If you mean the PiDrive cable, it's slightly different.
It has 4 connectors on it:
Micro-USB female - you plug the standard wall-wart power supply in here.
Micro-USB male - connects to the Pi to power it.
Full sized USB3 male - plugs into the PiDrive.
Full sized USB male - plugs into the Pi, for data transfer.
Basically the cable is designed to allow the same power supply to support both the Pi and the Drive, and to also act as the data cable between the two.
I have one and it works well, presuming your power supply has sufficient current supply for both. It's designed for simple convenience in reducing the amount of cables required whilst ensuring everything is powered properly.