If you have subscriptions then you must have a card on your account. That is what Apple says. We are not Apple so we cannot tell you why, only what the requirements are. Sometimes the money will be taken from your personal balance instead of the card, but the card has to be there.
https://support.apple.com/HT201266
"To make purchases from the App Store and use subscriptions like iCloud+ and Apple Music, you need at least one payment method on file."
If you can’t download or update apps on your iPhone or iPad - Apple Support
- "You might need to have a payment method on file, even if the app that you want to download is free."
Remove a payment method from your Apple Account - Apple Support
- If you have any active subscriptions, you must have at least one payment method on file other than Apple ID credit.
- You might not be able to remove a payment method if you have a balance due.
Sign in to your Apple Account - Apple Support
- "When you sign in to the App Store and other media services for the first time, you might need to add a credit card or other payment information."
How apps, content, and subscriptions from Apple are billed - Apple Support
- You can’t charge gifts to your Apple ID balance. If you send a gift or gift card, your primary payment method is charged.
- Some subscriptions might not be charged to your Apple ID balance.
What you can buy with your Apple Gift Card or Apple Account balance - Apple Support
"Some purchases might require that you have a credit card on file, even if you use your Apple Account balance to make the purchase."