Answer
The extended frequency range allows for a more faithful reproduction of the original sound.
Work Step by Step
Fourier discovered that any periodic wave pattern, no matter how complex, can be built up by adding sine waves of different frequencies. When playing back recorded music, using more sine waves, even those with frequencies beyond the audible range, increases the accuracy of the reproduction.
Incidentally, this is why some audiophiles with discerning ears still insist on playing vinyl records - they contain frequency components beyond the audible range, and beyond the capability of CDs to reproduce.
This is discussed on pages 398-399.
