It's much easier to comprehend the data from multiple windows if I can see both at the same time. Instead of switching back and forth just have them both open. Virtual desktop work very well too, but they're still improved with multiple monitors.
Depends on what you do. My setup is simple and I don't see how it could be improved with an external display - I've tried a couple of times but have always reverted to a single (laptop) display. Instead of switching between desktops I'd have to be moving my head.
There are certainly use cases where multiple screens have their advantage and I've also every now and then drooled over a 4k monitor, but then I'd lose the portability a laptop gives me (and/or suffer when constrained to it).
Information is independent in windows currently opened on my computer, seeing them simultaneously will only distract me with currently irrelevant information. Seeing the second monitor is another big if, I once had two monitors, but the second was too far away I didn't see anything on it and didn't bother to look.
