MailPress is the most powerful free self-hosted automatic newsletter plugin for WordPress, according to my detailed comparison of WordPress newsletter plugins. But and all that power can make it a little tricky to configure.
Having a newsletter on your blog allows visitors to subscribe to regular updates via email. An automatic newsletter simplifies the process so that all you have to do is post articles to your blog, and the newsletter to your subscribers is built and sent automatically with no extra effort from you. Even with the advent of RSS feeds and Twitter, email is still the easiest way of getting back in touch with people who have visited your blog before, and giving them a reason to visit again.