But it isn't just about the technical, is it?

The idea behind it is "headless commerce" -- a shop owner can operate single backend while diversifying the front-end channels from which customers are able to transact (marketplace, webshop, third-party, mobile app). It's a value-add service that all ecommerce sites will eventually move toward to retain customers.

With higher transaction volumes, provided customer data is collected and passed through, the shopkeeper can now access analytics for multi-channel targeting, marketing etc. Shopify can then sell all kinds of value-added services based on this data.