[McDyver]

If you already downloaded if from outside the app store, why do you need to use an in-app payment provider?

At this point it's just like any software on your computer, you can pay outside of it, and use a code or similar to indicate you paid for it.

It's only a matter of convenience to want to pay in-app. It can also be a lock-in strategy from the developer - and this is what this whole thread is about.