Why do I have to put my credit card in to download a App

If one or more of the following 6 options are true then you can't choose None as a payment method then the Billing information is needed. The "None" option will not appear...

1. If you have an unpaid balance or payment due
2. If you have a subscription that automatically renews
3. If you're using the iTunes Store or App Store for the first time
4. If you are not in the country or region you’re creating an Apple ID for
5. If you changed your country or region
6. If you're a part of Family Sharing

If you can't download, make purchases, or update apps, including free apps, you must update your payment method or add a new one. If you verified that a payment method is available in your country or region and you're still having trouble, learn more.

Simply said, it's a requirement to use the App Store or iTunes Store. Even for free apps. And if you look at almost ALL free apps, you'll notice a statement they offer "In App Purchases." Developers of apps often offer their apps for free and they are free, meaning you'll NEVER be charged to download or use the app, but In App Purchases are the ONLY way these app developers can make any money. They are not philanthropists. They need a way to make a living wage, so they hope something might encourage you to buy something. But you don't ever have to. And that is why Apple requires a valid payment tied to your account. Without one, you won't be able to download apps. But it will never cost you anything unless you make an actual purchase.