I never thought much of dual displays, or what I would ever use them for until I hooked my 20" ACD up as secondary monitor for my new 24" iMac. At first, I thought jeez, I have this huge screen now, did I just waste $20 bucks on an adapter? Then, I started to realize the possibilities, and frankly, I'm hooked!
Three things that I could no longer do with just "one" screen:
1. Run Aperture - it's designed for this, and it rocks having palettes on one screen, photo on other, or multiple photo's, whatever.. it's great!
2. Final Cut - LOVE having my Canvas on the 20" as a full screen viewer, and having SO much room to play with the other windows and stuff.
3. Fusion - Love running my work VM in full screen mode on the 2nd display. It pretty much feels like having two computers on my desk.
There are a lot more uses, but for me, I could no longer live without dual displays for just those 3!