If it's portable and doesn't require installation, then I can open it in the computers at the university or at work during a lunch break. Then work on the campaign a couple of minutes, save it. Then if I need to go somewhere on the weekend, I can take my laptop, open it and work some more on it. Then save it and when I come home, re-open it and work again on it. I don't have to install it in all of those different computers basically or any other computer in the future where I don't have administrator rights. Also, if I can't take my laptop, as long as there is a computer I can run it from the thumb drive.