The GTX 590 is Nvidia’s newest dual-GPU graphics card. In layman’s terms that means “two graphics cards in one.” They’re useful if your motherboard only supports one graphics card at full speed, as you get the benefits of “dual GPUs” with only one card. (That’s why I bought an AMD Radeon 5970 last year, because my motherboard only has one full speed PCIe slot.)
They’re also useful if you’re absolutely insane and want to have two, dual-GPUs linked together for a dual SLI or dual CrossFireX setup.
Not that any game released in the past few years needs that kind of power, but there’s still a ton of people out there who like to post their super-high 3DMark Vantage scores on this or that message board.
In related news, AMD released its newest dual-GPU card, the Radeon HD 6990, a few days ago. TomsHardware wasn’t too impressed, noting that it was as loud as a truck and not really any better than a pair of 6970s Crossfire’d together (physical space inside your case permitting)
But again, until we see games actually developed to take advantage of the PC—the only two that come to mind that will be released this year are Shogun 2: Total War and Battlefield 3—, buying these gargantuan cards borders on the “I have money to burn” side of things.