Using Visitor Stats 2, I can easily see how many visitors and sales have come from my Google Adwords Campaign.
I might place an ad with Google's advertising program, Adwords. I want to see all visitors who come from this ad, and see which buy my products.

In my Tracking area of Visitor Stats 2 (opens new window), I can see that I have only had 3 visitors so far, and but already made 2 sales ($100 total).
Remember, this is a demo, I wish I had that ROI!!
This also shows me which visitors came from that specific Google Ad, not just from Google (such as people just finding me in Google, rather than my ad). Tracking visitors separates them from general Google visitors.