digital out is there as some people prefer using a DAC circuitry better than what is used on the sound card. Though audigy is known to be moddable to sound quite alright, in its purest form it isnt really all that great, or such is the opinion of many enthusiasts. By having a digital out, they are giving you an option to bypass the inferior on-board one and spring for a better standalone DAC, or a receiver, amp that has a better DAC built-in.
Some "multimedia" speakers come with a digital-in, and often the quality of their DACs are not any better than audigy's, so I see it as a gimmick rather than a feature.
I am a headphile, Yeo's Monica 2 USB works fine for me
