Do you mean what are the benefits to having two? In the Edge browser, I can download on one screen and then drag it across to the other screen where I have the opened folder where I want the DL to go, it's fast and easy for me to do it this way. A lot of times I have the forum open on one screen and the gallery open on another screen.
In PS, I have my 12x12 page on one monitor and my styles, brushes, etc on the other screen, so everything is laid out neatly side by side.