Apple ID
Why when I want to download an app it asks for my billing information
iPhone 7
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This is a very common observation. Apple does not explain this occasional and inconsistent requirement. Enter a valid payment method to satisfy their requirement. If the item is free you will not be charged. You can also try to use your account without a payment method but you may still encounter the requirement.
Create or use your Apple ID without a payment method - https://support.apple.com/HT204034
If you can't remove your last payment method or use no payment method with your Apple ID - https://support.apple.com/HT203905
"Learn why you can't use your Apple ID without a payment method in the App Store, iTunes Store, and more."