The reason they do this is simple, e-mail is the Internet's killer application. There are enough e-mailboxes in the world for every man, woman, and child in North America to have their own. According to eMarketer , in 1998 there were 3.4 trillion e-mail messages sent in the world. That's 9.4 billion messages every day in the US alone. When the owners of those mailboxes are sent a message, it appears right there in front of them.
Quick turnaround equals quick results.
No driving Web traffic to site to get the message, the message comes to them. E-mail is direct marketing at its finest. E-mail is dirt cheap. Use it to promote new features, updates, new product offerings. Beyond the basic information-sharing function of a newsletter lies its branding effect.