I have a soundcard with SPDIF output. Isn't that as good?
Anyone who has been seriously looking at the performance of sound cards can tell you that what comes out of the S/PDIF output is probably not a very "clean" copy of the original. Many of the most popular soundcards perform all of their internal "processing" at a 48 kHz sampling rate, which is NOT the rate that most music (MP3's & CDs, for example) are recorded at. This means the music undergoes a sample rate conversion, which introduces a significant amount of distortion. Professional products that are designed to do this conversion correctly can cost thousands of dollars. Obviously your sound card isn't in that league. You can see examples of the problem by visiting our page.
Also, when you use coaxial SPDIF, you are connecting the computer's ground to your stereo system - not a terribly good idea. So the obvious question is, "won't the stereo-link have the same problem with the USB?" We've taken care of that by incorporating isolation circuitry to keep the computer ground separate from the analog output.