How can one of your external computer DACs improve my music?

An external DAC is essentially a very high end external sound card. Its components, and the implementation of those components, make it more capable than what might be built into a computer, not to mention the benefit of moving those sensitive components outside and away from the potentially noisy interior of your computer. All of this means that the signal your speakers receive will be clearer, with a lower noise-floor. Also, the signal will have a more pronounced dynamic range over your internal sound card.