They allow you to transport a signal(audio or video) over a distance without, or at the very least, little interference.
Speakers however, are analog driven devices and require you to convert the digitally transported audio signals, back to analog. You can't run an analog signal very far before the cables themselves start acting like antenna, picking up distortion/noise along the way.
All the audio on your PC hard drive is digital. Even your onboard audio chip has a DAC right before the signal is passed to the 1/8" mini jack.
You can maintain signal quality by keeping the signal digital as far as you can(if your speaker amplifier for your PC is not near your PC) you might want to use the TOSlink or SPDIF output to transfer the signal to your external DAC (presumably closer to your speaker amplifier). Because these are both digital signals, they won't lose anything in the cable(or gain anything for that matter).