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Monitors have gotten cheap, and developers can use the extra real estate. It is penny-wise and dollar foolish not to supply developers with ultra-fast computers and dual monitors. Every moment that a knowledge worker stares at an hourglass is pure wasted productivity. Having to work to manage all the overlapping windows on a cramped monitor also wastes time.
Multiple monitors allow you to write code on one and debug on the other. Or keep documentation alongside your coding. Having multiple monitors is just the first step, though, because you can also segregate your dual workspaces into a bunch of specialized views using virtual desktops.