Hooking up your PC to your DAC helps isolate your audio system from noise generated by your computer and results in a much better expierience. I know lots of people will gripe about mp3s being an inferior audio format, but I have some very rare songs in mp3 and I love listening to them thru my DAC, I find its unsampling capabilities are very benefical to mp3s.
I have some questions for some wizards out there, if someone is building/selling wireless method for sending spdif or toslink signals, I need it so e-mail me.
I do the same thing. My Sony VAIO came with a fiber optic output which is connected to a DAC. I think that the 256 and 320 kps mp3's sound pretty damn good, much better that using the normal miniplug analog output.