Amazon built this program to replace their old competition where they selected certain books to publish, and it was designed as a rolling enrollment system where people can load books at any time and have a chance at getting a publishing contract.
Amazon allows you to host giveaways of just about anything. You can give away your book (as an E-Book or Paperback) or things like new kindle devices or anything people might be interested in.
This is one of the sites that is hard to beat for authors. It has a great community of people who are constantly looking out for each other and helping bring everyone up.
I stumbled across WriteOn a long time ago when it was still new, and it evolved over time with a lot of ups and downs along the way.
Goodreads often feels like a bottomless pit where authors crawl to for self-flagellation. You can put in endless work reaching out in the community and get almost nothing in return.
Everyone swears by BookBub for results, and it is the undisputed king of the email newsletter industry.