Cost-free and open-access content can promote more than just a premium edition of the same content. Much like HTML websites, e-books can embed links; even javascript functionality is becoming available in e-book content. Publishers can use these types of functionality to generate revenue through advertising. A quick look at iPad or Android App Stores reveals a huge selection of free, advertising-supported Apps, including many apps that simply wrap e-book content.
In one scenario where this might happen, an author of a book series might produce an Open-Access electronic version of the first in the series. The free e-book could have embedded links or "in-app purchase" buttons for subsequent books in the series. Open-Access e-books might also be supported by contextual links and/or product placement; imagine a story featuring a sports car where the brand and model of the car are chosen based on support from a car company.
Another type of promotion that can be furthered by all types of free e-books is personal brand-building. It could be argued that Cory Doctorow's biggest payoff from his "With a Little Help" project was that it increased his fame and thus his ability to make money on appearances, commissions, and on the Boing-Boing website. (One story in the collection was a $10,000 commission.)