Here are both:

HOW:
the description above is pretty standard as far as how it is setup. Setting up another video card in your system will allow for multiple monitors (some computers can have 3 or even 4, even though I can’t figure out why they would do it, it is possible). The setup instructions are included with the new Video Card and should be followed, then once installed just plug a second monitor up to your new video card and follow the steps above and then you have dual monitors.

WHY:
Here’s the real question. They Why depends on you. Some people want a second monitor to show other people what they are working on. Imagine a graphic design artist showing a client a new project, and instead of both of them sharing a monitor there are 2 monitors, one for the graphic design artist and one for the client on the other side of the desk, and they both see the same thing. This is called MIRRORING (This is also used to hook up a digital projector for presentations). The second and more popular method is called EXPANDING or STRETCHING. (This is how it is listed above, if you wanted to mirror, just DO NOT check the “Extend my windows …” option. This will allow both monitors to work independently basically by doubling the size of your computers desktop. I am a software and web developer and so on one screen I will have code and design forms, and other will be a live version of the code being tested (or more likely a sports related website so I can keep up with the scores) but the idea is that both screens can work independently. It is also shown that computer people with multiple monitors work more efficiently that people with just one.

So there you have it. I hope it helps.