Why do I need a backup payment?

Thanks man, I only wish I used my money to buy those cards instead of more account money, but at least I know better now

Try checking at https://account.microsoft.com/services/ to see if it shows "Turn off recurrent billing" to the right of your subscription.

Either way, even if that is turned off, you will still be asked for a backup payment option when you purchase a prescription as the purchase will automatically turn that back on.

...I usually pay with my Microsoft account and I wouldn't mind this if I was a little short on my account balance but for these two times, my account exceeds the amount needed, taxes included. ....

May I suggest you switch to prepaid Gold/GPU cards instead?

The problem is: If you are overdue because a membership couldn't be paid with your Microsoft Account money, you will have to pay the overdue amount PLUS the amount you wish to use for the upcoming membership.

Since some stores also sell those prepaid cards cheaper than Microsoft's suggested retail price, you can even save more by buying those cards whenever you feel like it, instead having it automatically renew with your Microsoft Account money.

it's a form of backup, you can use a debit, credit or paypal card as another option, but you can keep as the main microsoft account balance, you can also cancel automatic subscription renewals to avoid payments

It's because you're purchasing a recurring subscription. Even though you have enough in your balance for the first payment, the system wants to make sure you have a way of paying for the next payment, even if you plan on turning off auto-renewal. If you
have enough in your balance for 2 payments, I believe it will let you pay without needing to add a credit card or Paypal.