Focus.

Say I'm working on Photoshop. I may need to use the palettes, tool presets, brush catalogue, alerts, etc, but then I may want to ignore them and focus purely on the image.

3D - I can put references on secondary screen and peek at them every now and then, but not have them distract me while modeling and texturing.

I can put tools like calculator, notepad and timer - all accessible, but all out of the way.