Customers that are issued a refund are at a pivotal moment in their customer journey. The refund flow allows you to engage with them to find out what went wrong with their order, express your appreciation and willingness to solve their problem, and to hopefully reactivate them as a customer.
Engaging with new customers is a must if you plan on building a long-term relationship with them. The Post Purchase flow allows you to just that. It should not be confused with an Order Confirmation message (that is sent separately by your eCommerce platform) but is instead an opportunity to begin sharing brand and product information, providing helpful tips or FAQs, and to eventually upsell and cross-sell the customer on other products in your store.