Because your product is a program and some of the features are paid. So you have a screen for the user to start/manage/cancel their subscription? It doesn't have to be some JS heavy thing, just that it will be a page in your app and you get autofill for free from the browser.
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When using my program, why would the user need to supply a password or a credit card number?
Because they’re logging in to your application and then buying a thing with a credit card?
Why would I write a program for this, when we were doing it decades ago with web pages that honestly didn't even require JavaScript to function?
Because your product is a program and some of the features are paid. So you have a screen for the user to start/manage/cancel their subscription? It doesn't have to be some JS heavy thing, just that it will be a page in your app and you get autofill for free from the browser.