It's cost effective
The big savings start when you consider there are no printing costs and you can get your message delivered for pennies per recipient.
Email generates an immediate response, instead of waiting for a subscriber to visit your site you get your message to them when it counts. The majority of your recipients will see your message in the first 24-48 hours.
Email makes it easy to segment subscribers using a variety of criteria like demographics and past campaign behavior (such as clicking a certain link). Ensuring your message goes to the individuals most likely to be interested.