More screen real estate is the obvious reason for dual monitors. When you're trying to edit a feature or long-form doc, you need that extra monitor for all those freakin' bins...!
For editing, the standard set-up is (for both AVID and FCP) to have viewing monitors and timeline on one monitor, bins on another. I have at least 5 presets for standard editing, FX editing, audio mixing, preview, etc.; with certain tools on the screen at all times.
The same idea can be applied to everyday use or other apps.
Someone mentioned that the best reason for dual monitors is so that one will used as a 'real' TV monitor. Wrong. A CRT monitor is used as a THIRD monitor to properly view the program, color correct, check titles and title safe, etc. Perhaps the person meant one computer monitor and one CRT monitor; but 2 computer monitors plus 1 CRT is standard for pros.