Though a few shopping APIs are some of the earliest examples we have, their popularity has continued to grow for a simple reason: there's an obvious revenue model tied to them. When developers use one of the 82 Shopping APIs in our directory, that can mean money in the pocket of the API provider. And, by virtue of various affiliate and revenue-sharing programs tied to many of these APIs, it can also mean money in the pocket of the developer.
Shopify has replaced its legacy Discounts API with the New Price Rules API. The API keeps what worked from the Discounts API, and greatly enhances those features with more flexibility and capability. One and five Shopify sales includes a discount of some sort, and Shopify wants to capitalize.