Why do you have to have a card with money on it to download free apps

Yep! Valid payment method is needed or money available on your account which can be done by a gift card even to download free apps. Even though the app you are trying to download may be free, it most likely contains in-app purchases where a payment method would be needed if you wanted to make purchases within the app.

Hello,

I recommand to read this article please

If your payment method has been declined in the App Store or iTunes Store - Apple Support (IN)

> scroll until this chapter please.

If you still can't make a purchase

• You may have an unpaid order. Find out how to pay an unpaid order.
• Contact your financial institution. 
  • You may need to ask your financial institution to allow international purchases on your debit or credit card.
  • If you want to know why a debit card, credit card or other payment method has been declined, speak to your financial institution. Apple doesn't have information about why the payment method was declined.
• In some countries and regions, your payment method may be declined because of changes to local law. Find out about billing and purchases in Europe or what to do if your card is declined in India.
• Still need help? Contact Apple Support.

kind regards,

The reason you need to have a payment method with funds available is because most free apps offer "in-app" purchases. That doesn't mean you ever have to buy anything in-app, but in order to download the apps, you must have a valid way to pay for something if you ever wanted to. Free apps are great, but keep in mind, they are from 3rd party app developers, who make no money unless someone makes a purchase. Supporting them and their creativity is a good thing so they will continue to develop and support those free apps.