DAC=Digital Analog Converter. You are using a DAC in your computer (onboard sound card probably) when you connect from the PC to your speakers but it will be a very cheap DAC and the reproduction of the analogue signal won't be as accurate as a more expensive DAC. Some people with pricey headphones will buy a pricey DAC. Otherwise you are coupling nice headphones to a crappy onboard sound card DACs which is a bit of a mismatch.