It's the simplest of push technology. It kicks the heck out of banner advertisement. You can be more descriptive with a text ad. Plus, I believe that the people who subscribe to one e-mail pub are more than likely to subscribe to another. Banner exchanges are fine too, but chose your friends carefully.
Attention spans are short.
Do what you can to get return visitors. Updating your site is key. Jim Romenesko of Obscure Publications notes that his visitor count was flat when he let cobwebs grow on his site. Finally, do your best to answer your mail promptly. It crafts your image.