In layman terms, rich pins help Pinterest get extra data related to your pin such as title, author, and meta description OR if it is a product pin it can be price, quantity, ingredients, etc.
This allows Pinterest to automatically synchronize data between your website and your pins each time you upload something. It also allows the Pinterest algorithm to talk to the Google algorithm and help your rankings.
They are not a necessity to succeed on Pinterest, but there is zero harm in adding them as they make your pins more discoverable and up to date on search.
Please check out Phase 1, Lesson 5 inside the Academy to learn more.