A trip to the mall takes a lot out of you. You've got to park, feed the kids, navigate the crowds, carry packages . . . you want to get what you need and go home.
Pinterest brings the mall to your couch. There's infinite variety and no pressure; the site lets you kick back with a glass of wine and "window shop" at will. It makes you likely to buy more, and that's why retailers love it.
Check out this Reuters article on why Pinterest is the new retail frontier.