It's human nature. As a species, we are naturally curious. So flipping over a postcard to see what's on the other side is natural thing to do. So a postcard with a picture on it can pack a punch far greater than it's small, small size would indicate. But as a way of letting people know they still had time to register and come to our seminar, the postcards worked wonders.
Postcards are convenient.
People will give them the once over even if they eventually throw them away. This means prospects get your message and then decide whether they are interested or not. All too often, no matter what you do, an envelope never gets opened. People are busy and are bombarded with over 2,000 sales messages a day. So it's no wonder some envelopes never get opened.