A DAC its acronym is DIGITAL to Analog Converter. Why to you need a DAC, because your headphone doesn't understand digital. The signal coming out of the headphone jack is analog and the conversion is already taken care of by the Motherboard's built-in DAC.
So why people seem enamored of adding an external DAC when the Motherboard already has one? Potentially better FIDELITY. If you decide you want an external DAC then you will get your signal from the Motherboard Optical (digital) output, and sometimes USB.