Why does it needs to put the billing information when I download an application?

This is a user-to-user forum; no one here has access to the internal policies that would be needed to answer a question that begins with “Why”. Without knowing the reason, Apple requires that there be a valid payment method in your account even to download free content. If you don’t have any apps with in-app purchase capability and don’t have any subscriptions you may be able to remove a payment method once the account is set up.