Get a Video card with 2 outputs - and set it up as a dual monitor with your O/S. or Nvidia software. It's pretty simple.
I got 2 monitors on my first build - and couldn't imagine not having it.
Right now - I'm watching a baseball game on monitor 2 - and typing this post on monitor 1.
Have you ever written a document - and done some web research for your project. You can now have both open on 2 screens at the same time. Or have the hep screen on one monitor - while your document is on the other document. OR - have your Web Browser opened in 2 different windows - so you can see 2 pages for research side by side - rather than having to toggle between tabs at the top of your page.
My 2nd monitor always has a Stock ticker application and post it note via Windows b/c that suits my needs. You might have things that interest you - that you'd want on your second monitor.
Basically - You have 2 screens - and can run 2 full screen applications at the same time. It functions like having an elongated screen to move data from one screen to another - but the page responds to your monitor individually < like maximize screen to 1 full monitor > You can drag anything from screen 1 - to screen 2 via your mouse - and vice versa. You can easily live without one - but it gives you more productivity to have 2.
I had a free 15" monitor that wasn't being used - and just added it b/c my Video card had 2 outputs. I can't imagine having only only one monitor now.
If you are starting from scratch - ideally get the same resolution for both monitors. Going from a 1080p to a 15" 4:3 monitor - makes the windows drag and drop a bit awkward in the 15" monitor - but I can adjust things to get it working fine.
If you have interest in 3 monitor setup - there are some games that take advantage of Newer Technology in the ATI 5xxx series cards called eyefinity for 3 monitors on 1 card.
FYI - there are other ways to set up your dual monitors - but what I use is a simple example.