Charlie Huston used one of his books (no longer free) to get me hooked on the rest of the series. Get one free, buy three. Backwards but effective.
Another: To spread an idea you believe in (where money is not the object).
And: To create hoopla for a new book launch. Josh Bernoff is doing a freebie with his new book, just this week. (Sorry, US only–publishing rights are largely a pre-digital artifact).
When the marginal cost of the interaction is zero, the marketing opportunities of spreading an idea increase dramatically.