This is linked in the 2nd sentence in the getting started section that is linked from one of the two biggest call-to-actions on the homepage: http://docs.vagrantup.com/v2/why-vagrant/
Thanks for pointing that out. I still think it would need a concrete example or two. The text is very abstract, long sentences with multiple consecutive adjectives describing how easy it is to use and how powerful it all is. A lot of fluff really. You're trying to sell the idea, not describe how it works.
I'm SURE it's a fine tool and I'm just trying to help you communicating it better. :)
I'm sorry you couldn't find that!