The biggest reason is that computer monitors take up an inordinate amount of space on the average desk. Even with the advent of flat screen monitors, it's not unusual for a computer to take up half the depth of the average user's desk. We have come to accept this as the computer has become the focal point of our workspace, but there is a better solution.
A computer monitor attached to a monitor arm takes up a bare minimum of desk space. Fitting onto the back of you desk, a monitor arm allows you to fit papers or other desk items neatly underneath the monitors themselves, while allowing you to manipulate the position of your monitor quickly and easily. You won't find yourself squinting at your computer screen (a common office ailment known as eye strain) or hunching over your keyboard to see a smaller font. Your monitor (or monitors) will now come to you.
You'll be amazed at how much this frees up your desk space, and everyone can use more of that. It's rare to find a desk at the office that isn't covered in stacks of paper or literature, and at home, many of us have makeshift home offices that erected in whatever space is available, which often means a very small space and an equally small desk. Humanscale monitor arms go a long way toward make the most of your desk. They provide flexibility, convenience, and they turn your office into a more effective place.
Monitor arms can be extremely beneficial from an ergonomics standpoint. The greatest advantage they offer is their flexibility, which allows the computer monitor to be viewed at an optimal angle no matter how the user happens to be sitting. The positioning of the computer monitor can be a significant obstacle for an office worker, even after that user has invested in an ergonomic office chair.
To demonstrate this point, try this simple exercise. find the most comfortable seating position you can at your desk that lets you to type, but do it without paying attention to your computer monitor (pretend the monitor is not even there). Once you've found the most comfortable position possible, see if you can clearly read your computer monitor. If the answer is no, then that means you're going to have to sit in a less than optimal position to effectively use your computer.
The fact is, if it weren't for our computer monitor, most of us would recline more than we normally do. As a workaround, we could just move our monitors closer to the front of our desk, but that would leave even less workspace for papers, books etc that need to be placed close to us. The end result is a seated posture that's not ergonomically sound. Fortunately, monitor arms offer an easy ergonomic solution.
Flexibility, ease-of-adjustment, and ease-of installation are all factors. A truly ergonomic arm should make it easy to adjust the height, depth, angle, and orientation of the computer monitor. The goal is to be able to meet as many different positions for the screen as you can imagine.
Of equal importance is how easy it is to make these adjustments. If you have to turn a screw, twist a knob, or (the worst) get up out of your chair to make these adjustments, then that is a design flaw. Of course, every monitor arm will require some kind of action on your part, but some are better than others. Generally, you should be able to make most adjustments with one hand while remaining seated and without any strain.
Not to be overlooked is how easy it is to install the monitor arm to your desk. Installation can range from very easy to very difficult, so investigate how much energy you think you'll be exerting during the installation before making any purchases.