How beneficial are dual monitors?

I have been learning iOS app development. I want to create an app and was wondering if i should get another monitor to go with my iMac so i will have extra screen realesate. I've been looking and so far really liking the asus mx239h monitor. It kind of reminds me of my iMac with looks. I have been thinking is this the course i should take or should i look at other things like an external hard drive or there things that i could benefit from. How have dual monitors influenced your work?