All major book sites also tend to have free books also. You can normally do a search or pick a category and sort price low to high. Often when a book is free on one site, it will become free on the other sites.
The allows Amazon Prime members to "check out" your e-book for free (members can only check out one eligible title per month). Obviously, being able to offer your book for free to thousands -- or potentially million of customers -- increases the odds you'll "sell" more books. And what's nice is that even though people may not being paying to download your book, you're still getting paid -- and pretty well, according to Amazon.