why should i pay for apps even though they are free in the apps store?

Are you asking why a payment method is being requested? Apple sometimes requires a payment method to be on file even when downloading free apps.

Log into your account in the App Store/iTunes and you should be able to change/verify it there. Or you can click the blue Edit payment methods in the article below. When signed in and the page loads, you will be on the credit card page. After updating that, click the Go Back button at the lower left and you will be on the address page.

Apple ID payment information - change. 

If an app is free, you don't have to pay for it. It kan be that a free app has in app purchases tho, but you decide for yourself if you will buy those