More screen space, really. I tend to use my second monitor for apps that I want to be able to quickly find at a glance. For instance, on my second monitor I have SpeedFan up to monitor temps and fan RPMs, Task Manager to monitor resource utilization, Winamp, Ventrilo, and notepad with the server names and IPs for all of our servers.
I also use it a lot if I'm doing stuff like merging documents. It's really helpful to be able to glance at each document at the same time instead of switching back and forth.