Not sure if this is what you mean, but there could be multiple apps installed that write to the device photo library. You may not want the developer of one camera app to be able to access all photos on the device.
But this raises a related point about how frustrating Apple's APIs are here: When an app is granted the "Write to photo library" permission by the user, it can only write. It can't read back what it's written, ever. You might expect that writing to the library might return a token that can be used to read that photo back. Nope.
Android, for all its faults, does a much better job here. The OS keeps track of the app that wrote the photo -- and that app can read that photo indefinitely, unless another app edits that photo (and thus becomes the owner). A much better design.
On iOS, to read back photos from the library, you have to ask for the "All photos" read permission, which few people will grant you. "Why does my camera want to read all the photos on my device?! Deny!".
And just like that, you can't compete with the built-in camera which shows thumbnails of recently taken photos and allows you to swipe through them.
Apple has no incentive to fix this either, because their own apps bypass this permission system.
