Some people would like to use their Empegs with external DACs, either at home or in the car. Digital audio transmission, especially optical, is more immune to noise than analog. Also, depending on the quality of the external DAC, you can improve the quality of the sound by virtue of higher quality equipment. If all signal sources are input through a single DAC, the output level will also be more closely matched across signal sources.

Hope I'm not explaining the obvious,

Stu