It is useful in some situations when we don't have access to the rear camera to scan. For example, when i'm in a car and mounted my phone, and then if I want to take a scan of a bill or receipt.
Well, why would you have to do that in your car and not wait until you can take your phone out of the cradle? How hard is it to uncradle the phone anyway?
May be it's not so hard to make build in feature, as example Camera App scan QR-codes but not do it on front-facing camera, but it's so specific scenario. I can suggest a workaround: use front-facing camera in camera app and use 'smart select' or 'circle to search' feature - thay both can scan QR-codes from screen.
