Rich pins take the metadata that you use to optimize your website’s google SEO and depict that data along with your pin.
Therefore, rich pins make sense for the following type of pins: recipe pins, product pins, app-based pins, and article pins (e.g. for blogposts).
If you do not have metadata on the pages that you want to promote through Pinterest, because you are for instance simply aiming at social media promotion, you don’t need rich pins.