An extra 3840x1200 pixels
It's a relatively affordable way to add a large amount of extra pixels to your desktop. You can find a 22-inch TN Full HD monitor with DVI for around £95, and if you add the £117 for the Matrox DualHead2Go ME you have an additional 3840x1080 pixels for a little more than £300. If you get a fancier IPS display you'll obviously spend more, but it's still significantly more affordable than a 2560x1440 27-inch monitor. The Matrox DualHead2Go sees two monitors as a single large desktop, and not as two individual screens.
With the DualHead2Go ME or SE you can create a single large desktop.
Matrox's approach has both advantages and drawbacks. Even if a program has not been optimised for very large work spaces, it works flawlessly here. This is true for software such as Excel, but also for video games. And that's where the problem comes in as well, since everything automatically adjusts, it's not always practical to have a huge screen.
Maximising a window isn't always practical with such a large desktop.