A customer forgets their discount code, emails you, and waits.
The usual fix is a partial refund. Merchants in the Shopify forums have documented the fallout for years: tax reports that overstate the sale, refund metrics that look worse than they are.
All of that for a code the customer had the whole time.
The cleaner route is letting them apply the code to the order they already placed, from the order page, before fulfillment. The discount lands on the order itself, where it belongs.