If you look at the HTML code for an email, you will see that it is made up of nested tables. The email itself is a larger table, each block (the greeting, introduction, coupon, etc.) is a table, and each block contains tables. Constant Contact
The reason for this throwback to the 1990s (i.e. using tables for presentation) is because of the way that various email readers handle CSS and HTML. Some email readers are way ahead of others however some are just plain archaic. Below is an example of how the same email looks in two of the most popular email clients; Gmail and Outlook 2007. Notice how the same content and style sent to two of the biggest email readers just looks so different.