Yup, my onboard audio was pretty noisy, so I needed the DAC to clean it up. Even so, I get an improvement on that same DAC from my desktop PC, which has a decent dedicated sound card to compare to.
This could be sampling bias, however, because my DAC (FiiO e7) is also an amp. Then again, sound cards have built-in amps as far as I'm aware, so it should, theoretically, still be comparing apples to apples.
I dunno, like you said, it's all pretty confusing :)