Why I need to pay money?

Hello yon,

This article explains some reasons for problems downloading apps If you can’t download or update apps on your iPhone or iPad - If you can’t download or update apps on your iPhone or iPad - Apple Support

Note the line saying, "You might need to have a payment method on file, even if the app that you want to download is free."

Because many apps are not free. The developer charges for them. Even if the app is free you must provide a payment method before you can download a free app. That’s policy.