Rich pins are to Pinterest marketing what rich snippets (Schema.org) is to search marketing.
Adding metadata to your pinnable content such as pricing, Buy This calls-to-action, descriptions and stock availability gives Pinterest users more information than just the visual.
But one of the most interesting perks of rich pins is that Pinterest notifies any user that's pinned your product by email when the price drops. Even if the user has never been to your site, never opted into your email program, never heard of your brand -- that pin-pointed email is in her inbox. True one-to-one marketing, down to the product level. (And for Gmail users, that message lands in the Social inbox, not Promotions).
Strategic price drops for the most popular pinned products in your catalog is a sneaky way to spur social sales.
Retailers like ModCloth, Walmart and Target pull popular pins into merchandised areas of the e-store using the Pinterest API. This functions as an alternative to a wishlist, with a couple perks - Pinterest will send the email once the "Pin drops" (may have a higher open rate coming from the social network than retail email), and the user does not have to log in or create an account to save to a wishlist (reducing friction).
To take advantage, visit Pinterest's developer portal.