Sometimes all it takes is a little flesh to boost a movie's box office. A few obvious examples come to mind.
Now, American Beauty was a brilliant film that didn't need to use sex to sell it but what the hell? You've got a scene with a naked Mena Suvari covered in petals, you're not going to waste it - hey, pop it on the poster. Most posters just featured her belly button and a single flower but it's better than looking at Kevin Spacey's mug on the side of a bus, right?