Doing so unlocks a number of important marketing perks, including the ability to promote your book for FREE for up to 5 days. (udemy.com)
There are a number of sites that allow you to promote your book for free while it's available for free download. (janefriedman.com)
Before Amazon's PPC, Amazon promoted and still promotes books for free in a via the «Customers Who Bought This Item Also Bought...» section, of book sales pages. (firebrandpublishing.com)