The purpose of a DAC (Digital Audio Converter)/ sound card is to convert digital signals as processed by a PC into an analog signal that can be played on speakers/headphones. An analog is signal is a continuous signal, and small variations in the signal while transmitting the signal can significantly affect the audio quality. That is why high end audio equipment always make it a point to advertise their Signal-to-noise ratio. An integrated sound card usually consists of "low-end" components, as a result it tends to usually have a very low Signal-to-noise ratio (typically around 80dBm) where as good DACs have much higher Signal-to-noise ratio. A digital signal consists of zeros and ones, which are represented by voltage variations. Typically 0 V represents '0' and some value like 2v represents '1'. But this is just theory, in practice there is quite a bit of leeway, anything less than 0.5v might be considered '0, and anything greater than 1.5v might be considered as '1' and the in-between values are ignored. Thus a small variation will not affect a digital signal. So as Gannu has so masterfully explained, going via the digital route effectively bypasses the sound card, and instead the digital to analog conversion is done by the inbuilt DAC of the Z5500 which is certain to be much better than the integrated sound card.
