If you have ever been to the trading floor of a large brokerage company, you have seen some of the most advanced workstations around. Traders often use UNIX or NT workstations with two or even four monitors so they can display dozens of windows simultaneously. Multiple monitors have also been possible on the Macintosh for several years, and are often used by artists and editors to give the computer more screen real estate.
You might want to have multiple monitors for the same reason that you might like to work at a big table sometimes. For example, when you are doing your taxes or working on a big science fair project, it is nice to have several books and four stacks of paper open simultaneously. It works the same way on the computer. For certain tasks, it is helpful to have lots of open windows that are all visible simultaneously. This is especially true for computer programmers working on large projects, artists and editors cutting and pasting from multiple sources, etc.
It turns out that Windows 98 comes with multiple monitor support that allows two (or more) monitors to be used simultaneously. To try this feature you need a second graphics card and (obviously…) a second monitor. It makes things more symmetric if the two monitors and cards are the same, but they don't have to be. You can either place the monitors side by side to create a wide desktop, or you can place one on top of the other. With three or more you can arrange them almost any way you like.
Here are several useful links if you would like to learn more. The first link shows some nice screen shots: