Why do I have to have a payment method to download things

You must have a payment method if you download any apps that have in-app purchases, or that have subscriptions. You also may need to have a payment method to verify your identity. If this is the only reason you may be able to remove it later.

Also, if you DO have a payment method, it must be a valid one that is verifiable with the issuer. You can’t leave an expired payment method on your account.

See this Apple article:

If you can’t download or update apps on your iPhone or iPad – Apple Support (UK)

👉 You may need to have a payment method on file, even if the app you want to download is free.

Free apps may have in-app purchases that require a payment method to be available.