Why do I have to add a payment method to my iPhone to download apps. How can I turn this method off

Click on the link below, you can

• View your purchase history

• Cancel a subscription

• Request a refund

• Verify unfamiliar charges

• Add or remove a payment method

Subscriptions and Billing - Official Apple Support Subscriptions and Billing - Official Apple Support

You can't turn it off, you need a valid payment method on file with Apple for purchase, even if they are free because of in-app purchases.