Why do I need to add a payment method to download free apps on iPad?

Hello,

You need at least one payment method to make purchases from the App Store, per reference to the following article: Add a payment method to your Apple ID - Apple Support

If you are not using any Apple Subscription services, that payment method can actually be left blank - you will need to provide a valid billing address though - reason being: not all applications are available in all countries. The valid billing address is needed to validate that the country in which your App Store is connected is a country in which you do reside in. Hope this helps.

Yasamean