According to the company̵
The new App Store code feature gives developers more control over discount prices on iOS devices. It also encourages developers to push for App Store subscriptions, which is profitable for Apple. Subscribing to Netflix through the App Store ensures that Apple will lower your monthly fee. Apple cannot do this if you log into Netflix using the browser.
Either way, Apple says its offer codes are unique and alphanumeric. Electronic offer codes from emails or pop-up notifications can be sent directly to the App Store, where you can redeem the offer, ignore it or view the offer’s terms and conditions. Developers can also allow code to be redeemed in their apps, streamlining the process for discounts and other offers.
Customers can use these codes on the web, in the app store or directly in apps. Offer codes expire a maximum of six months from the creation date. Customers can redeem multiple offers for a single subscription, depending on an app developer’s offer code configurations.
Source: Apple via The Verge