The USB3 is an audio interface that uses the common USB connection to get audio to and from the computer. This USB connection allows the USB3 to transfer 2 channels (Left & Right) of audio to the computer (RECORD) and at the same time transfer 2 channels of audio (Left & Right) from the computer (PLAY) at the same time. The USB3 also receives all of its power from the USB connection.
Imagine having 5 cables connected to your computer: one for each the Left & Right Input; 1 each for the Left & Right Output, and an AC power cord connected to the wall. This is the functionality that the one simple USB connection provides.