The goal of this project is to allow people to receive postcards from all over the world, for free.
Well, almost free!
The main idea is that: if you send a postcard, you will receive one back from a random Postcrosser from somewhere in the world.
Why? Because, like the creator of the site, there are lots of people who like to receive real mail.
The element of surprise of receiving postcards from different places in the world (many of which you probably have never heard of) can turn your mailbox into a box of surprises - and who wouldn't like that?