Three uses for a free Kindle book
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.