In the second paragraph (though really, its just a statement...) on the preferences page -- no need to navigate to another page, and they tell it to you in plain english. Once again, you have to opt-in, so if you opt-in without knowing what it does it's your own fault and you're being a dumb user:

When you enable GhostRank, Ghostery collects anonymous data about the trackers you've encountered and the sites on which they were placed. This data is about tracking elements and the webpages on which they are found, not you or your browsing habits.

Online marketing companies need better visibility into real-world applications of their technologies and those owned by their competitors. GhostRank data is sold as reports to businesses to help them market to consumers more transparently, better manage their web properties, and comply with privacy standards.