We have long since passed the age when discrete sound cards were a de facto part of a Desktop shopping list. Now, one can enjoy excellent digital audio without a specialized accessory stuffed inside your desktop PC. All you have to do is connect a cord.
Of course, there's a bit more to the world of PC home audio than that. But we'll start with the basics: The ports. If your desktop isn't chiseled out of stone, odds are good that it comes with at least three, one-eighth-inch, analog audio jacks on its rear (in layman's terms, the skinny plug connector). One's green, one's blue, and one's red. Barring a switcheroo by your system motherboard's manufacturer, the first is the only output you get, and it's exactly where you'll connect a pair of stereo speakers or headphones. Red receives input from a microphone, and blue receives input from all other devices -- like a cable running out of the headphone jack of your iPod, for example.
Fancier motherboards or discrete sound cards typically up the ante with ports for up to 7 speakers and one subwoofer: an orange connector for your center speaker and subwoofer, a black connector for your rear speakers, and a gray connector for your side speakers. You might also find larger, circular connections for a coaxial SPDIF cable or a square-shaped, port for an optical SPDIF cable, emitting a red glow.
A SPDIF connection allows you to pass a digital signal through your soundcard off to an external audio device, like an audio receiver, for decoding into a multi-channel array of sound. You normally can't perform such a feat using your sound card or motherboard's preexisting 5.1 or 7.1 analog ports, as conventional receivers don't have the corresponding inputs.
Similarly, conventional receivers don't come with the same analog ports as featured on the rear of your motherboard or sound card: You're connecting your home speaker setup using stripped wires that are attached via binding posts on the rear of the device, not fancy analog plugs.
And we now come to the $10,000 question: Do you even need a discrete, aftermarket sound card if your motherboard already comes with some of the aforementioned connection options?
It depends. If you're just rocking a two-speaker setup that you use to play some of your favorite tunes, probably not. If you're looking to upgrade to a multi-speaker setup, you might consider investing in a discrete card for the additional ports or software features it provides. If your motherboard-based audio can't simulate a 5.1 surround out of a two-channel audio file, that could be a deal-breaking factor for you. And if you're intending on making your desktop PC the centerpiece of your home theater setup, a head-nod for more advanced connections that fit your receiver or speakers' inputs might be in order.
Will a discrete sound card automatically make your tunes sound better versus a motherboard-based chip? No. Will you gain a huge boost in a game's frame rates by offloading sound processing to a separate device? Nothing noticeable. What you will gain is the ability to do more with your system, but the utility is going to depend on your acoustical needs. Be sure to keep your use case in mind when you're shopping for a new desktop.