In the past, I've mainly used two monitors simultaneously so that I can read something no one screen while I'm typing on the other screen. When you have a laptop connected to an external monitor on "extended desktop" mode, you can multitask and get quite a bit done.
Recently, I've had the opportunity to test the Lenovo ThinkCentre M90z that has a 23" ultra-wide screen. In fact, the screen is so wide that I can work comfortably with two windows open, side-by-side. The M90z allows you to connect a second monitor if you require it, but I've found that I really don't need a second monitor when I'm using a primary monitor that's so wide.
You can purchase LCD computer monitors that 27", 30" and even larger. Once you get something that big sitting on your desk, you can probably open 3 or 4 windows and stack them next to each other so that you can truly multitask.