Firstly, it is free to publish on Amazon. You are only charged when you sell a copy of your book. Also, you can reach a much wider audience all over the world, while minimising your costs and therefore maximising profits. Amazon also help you through the process, and you decide to publish your book, without a publisher rejecting your idea. There are many examples of books that have succeeded as they started on Amazon.
Traditional publishers had so much control over the market prior to Amazon that there was less and less opportunity for new and unpublished authors and writers to get started. With Amazon many have been able to find success by self-publishing and the industry has been changed for the better by providing opportunities to many.
Yes, it is worth the time and effort of writers to publish on Amazon. As the largest book seller in the United States, and possibly the world, Amazon provides a level of exposure that an author may not receive otherwise. There are drawbacks for authors publishing on Amazon, but the fact remains that their work will be viewed by many more people than if they went through traditional self-publishing venues.
Self publishing is often the way to go for most writers who are just starting their careers. Getting a book deal is extremely hard to do nowadays and publishing on Amazon is a great way to try and build a fan base. This option brings many great books to readers who would have otherwise never seen the light of day.
On the positive side Amazon provide a great opportunity for authors to actually see their manuscript turned into a physical book. It is a great way to bypass the often impenetrable wall that is commercial publishers. Many of the books published via Amazon are of great literary merit. Unfortunately, as it is relatively easy the majority of the Amazon books are pretty terrible. Because of this there is a massive bias against any book published via Amazon. Bookshops won't stock the books, reviewers won't look at them and commercial publishers will not speak to you once you've gone there. It's a mixture of snobbery and the conventional industry fighting back against online sales. You cannot get a publicity for your work and with Amazon's latest ban on any reviews unless the reviewer has spent over $50 means that most books published via Amazon just sit and rot with no sales and nobody knowing they exist.