Your sound card converts the digital files from your computer into an analogue signal that you can hear through your speakers. You can not hear the digital file without it being converted, so the idea is to get the best converter possible to make sweet, sweet music - hence the expensive hardware such as sound cards.
Having your soundcard output a digital signal will mean that you're passing the file to something else to get it converted, which makes the soundcard pretty much act like a passthrough.
Digital signals, i.e. your optical out, do not need special hardware as it's merely the transfer of data. The optical out from your motherboard will be just as good as the one from a sound card.