I recently upgraded to a larger screen, a 30" 2560x1600 Samsung LCD panel. I'm not sure a screen this large is ever "necessary", but our GUI development environment takes up so much space, that my smaller screen was almost unusable. When we work on our WPF screens, we usually have five different panes open, with two main panes sitting side-by-side -- the GUI representation of the screen and the XAML code-behind where we write the screen code.
With my smaller 21" monitor, I could only see ten lines of code at once. With the new monitor, I can easily show the GUI and code-behind next to each other, at least 50 lines at once, making it much easier to debug and add features.
FinFolio follows the same paradigm, letting you put different panes beside each other. If you have a larger monitor, you'll have a richer user experience...