Email is one of the best ways to keep users coming back to your site. I’d lay out the arguments, but Patrick McKenzie already has. Go read that article if you doubt my statement.
If you generate a lot of content via your blog, then an email campaign that pulls from your RSS feeds is a great way to generate newsletter content. If you have two sources of content–a content blog and a link blog, for example–you can have the newsletters pull from both RSS feeds and have them feed different sections of your newsletter.
I am a big fan of mailchimp as an email delivery service (they have a great free plan I’ve used multiple times), and was ready to roll up my sleeves and use their API to read from a set of RSS feeds and generate a newsletter from that feed. But, luckily, they’ve already implemented RSS email campaigns. If you want a monthly newsletter and were confident of new articles in your RSS feeds, you can schedule the newletters out for a year, and know that you’d be sending out topical fresh content every month. (Note that if you didn’t have new articles, the newsletter would still be sent if you didn’t intervene. And I can’t think of something that would make an internet company look dumber than to send the same content twice in an email newsletter.)
I’m aware of solutions like paper.li that allow you to aggregate multiple sources of content in a pretty package, but mailchimp seems to give you more control and can be used for non RSS email campaigns as well.
Take a look!