Yes, there's a certain coolness about having multiple displays strung across your desk. But it's the way that multiple monitors can enhance your productivity that make them useful to designers and others. If you need concrete ways that a dual monitor setup can improve your working (and playing) environment, consider these scenarios that utilize that expanded view.
If you typically run multiple programs at once, such as email, calendar, instant messaging, and graphics software, multiple monitors make it easier to see what's going on without totally interrupting your current primary focus. While you're working intently on graphics for your current project on one monitor you can glance over at the other monitor and keep an eye on incoming email, newsfeeds, or that all-day-long-every-hour-or-so IM conversation you've got going with your mom or the guy in the next cubicle.
With some programs you can lose your work behind all the floating palettes. Sure, you could minimize or close some of them but if you know you'll be using them at some point during that session it's easier to just slide a few of them over to your second monitor while you do your work on a less cluttered screen.
A typical desktop publishing project involves three or more applications: word processing, graphics, and page layout. Instead of tabbing between windows or constantly minimizing and maximizing windows you can have your page layout program open on one monitor with your word processing and graphics programs open in side-by-side windows on the other monitor. As you're importing and placing text and graphics you can easily switch back to the originating program to tweak or edit the original text files and images without playing hide-and-seek with your work.
One of the draws of desktop publishing software is that it allows you to try out different arrangements of text and graphics easily before committing a final layout to print. With multiple monitors it's easy to throw up different layouts on each screen for a side-by-side comparison. Have a layout you like but you're wondering about a few possible changes? Put a version of it up on each monitor then tweak one of them and see how it compares to your original. Or, if you're trying to match an existing layout, put it on one screen while you try to match up the elements on the other screen.
Are you using new, unfamiliar desktop publishing software or trying to learn a new task in your graphics software? With dual monitors you can have the program open on one monitor and use the second monitor to hold its help screens, online tutorials, or video tutorials. No more switching windows or excessive scrolling back and forth.
Whether you are working alone or working collaboratively with others, it's easier to compare two versions of a document side-by-side. With two monitors you don't have to spend a lot of time trying to size document windows just right for a side-by-side view. Just pop up one on each monitor.
While it's generally more effective to proofread a printed document, a lot of proofreading can be done on-screen. Dual monitors makes the task a little easier. Open the original document on one monitor and a duplicate on the other monitor. As you proofread and make changes on the copy you can see how your corrections/changes affect the layout without altering the original. Once you're happy with all the corrections save the corrected version as the new and improved (and proofed) version.
Sometimes what's happening on both monitors is equally important. If you find yourself attending online conferences, having the video over on one monitor keeps all the action in view while taking notes on the other monitor. I often have my text editor open on one screen while using the other screen for online research that includes surfing, reading PDF files, watching videos, and keeping an eye on newsfeeds.