In a word, screen real estate.
Okay, that's three words... but that's the principle.
Some people use it in Photoshop or AutoCAD, with the main window maximized on one monitor and all the palettes on the other. It's handy for really
large spreadsheets, too -- it reduces scrolling and improves visibility. I use it myself when I'm building reports (generally in PageMaker) from multiple document sources (apps like Word, Excel, Mathcad, Axum, Photoshop and the odd ASCII editor or DOS window here and there -- and yeah, I'll have all these open at once, which takes two 21" monitors to handle easily).
My friends who are into coding speak well of it, too; something about code on one monitor and debug on the other... but I don't code, myself, so I don't understand it all.
But I find that it's as useful to me as dual CPU's, and from me that's saying something!