Email marketing has many advantages:

1. It is very inexpensive to deliver
2. The customers can get to know you personally, building trust
3. You can tailor it better than any other direct mail piece
4. You can embed surveys to know your customers better
5. BEST OF ALL you can track all activity and conversions

These are some of the top benefits, and there are more.  A company can realize these benefits if done correctly.  I have one ecommerce client that sees an increase in sales of thousands due to each newsletter.  We know this because it is directly measurable with the email program and website analytics.  Can you say that of a postcard campaign?