Postcards

You know postcards? They're those pretty pictures that people on holiday usually buy and send them to friends to say "hey, I'm in this wonderful place and you suckers are stuck at home doing whatever you're doing". Well, maybe not exactly that but it's usually what I think when I receive one of those postcards. And what's the deal with postcards anyway? Why do you write on the back of it for the whole world to see and post it to your friends?
Whatever! I was browsing in Borders yesterday and I came across this pack of postcards with a difference. Take a look at this.

Sorry about the quality of the picture. I was trying to take a picture AND put as much distance between me and these incriminating postcards as possible in case someone walks past.
So what IS the deal with these postcards? When do you send them and to whom do you send them and why do you buy these in the first place? I suppose if you're gonna send this you'd have to find an envelope for it. And what's the message behind the postcard? "We tried this amazing position yesterday and we thought of you suckers who don't know how to do it"??