Most desktops can run multiple applications at once, but you're limited by having one screen so having to swap between the two or lower the window size.
By having two screens, you can simply move between two applications by moving your mouse across, which is useful when using one for the other.
For example, I have access to key systems (databases) on one screen and my usual desktop on the other, so I can look and work on both simultaneously.
There's one lad in our office who requires three screens because of working between two key systems and his usual email or word processing.
This…when working on a lecture on one screen so much easier to have the reference/source on the other.