With the lack of ethernet in Apple current MacBook models, the DualRole provides ethernet connectivity by connecting to a computer's USB port. And since the DualRole has additional USB 3 ports, you don't give up an USB port on your Mac.
The DualRole runs off of the power from a USB port, but Kanex also allows you to connect a power adapter, in case you connect devices to the DualRole that require more juice.
The DualRole is $69. The device is on display at Kanex's booth (#630) at Macworld/iWorld.