... if you know how to use virtual desktops and shortcuts. You can't look at two screens at a time, anyways.
One use case I can understand is having a 2nd monitor for checking stock prices or checking for a certain event. Other than that, I don't see how it contributes to "productivity" while working or coding for example.
P.S: Tiling WM users may understand this post more
It kinda makes me disturbed to have something bright in the corner of my eye when I concentrate to read text. And even though I use tiling WM, most of the time I only have one window at once, rarely two windows split horizontally, but no more.