Quote from the movie Pearl Harbor: "Hey. We got a visitor."
The main goal when using a QR Code in a marketing campaign, or just running any other advertizement campaign is to generate interest in you product, and get visitors in your store, or on your website.
In a store you will get a variation of visitors, from the just browsing to the buying type. On the internet however there is also another type of visitors.
Some of these visitors are the search engine BOT's that scan your site, but there are also various other non organic visitors on your website as result of a campaign.
A good example of this kind of visits was mentioned in the The QR Code Gamble blog entry.
These non organic visitors could accumulate to a very high number of hits. When running a campaign and using a service that runs on a Cost Per Click (CPC) method, you could also be paying for these non organic visitors.