Whenever i download from app store , payment has to fill. Why

Because a verified payment method is required to be on your account to make use of the App Store services including downloading free apps.

If you don't have one, you must add one and verify it. Free Apps will not charge the payment method, but apps that have free trial will renew into a paid subscription at the end of the trial unless cancelled, and will need a payment method to charge for the subscription.