Why is my App Store asking for my credit card info even though I have money in my Apple account

I was trying to buy something for a game, which I was able to buy previously with my Apple money. But I added my card info to buy something quick and removed it afterwards. Now, I have money from an Apple iTunes gift card in my account, and no credit card, and it says I can’t buy anything because I don’t have a credit card added. Why would that matter if I don’t want the purchase charged to my credit card, but rather I want it to just use the money from my Apple account?