Redownload apps

Why do I have to put in my to redownload a app I had

Do you mean why do you have to put billing information, when you want to redownload an app that you ever had?

If so, yes, you need to have a payment method on file, even if the app you want to (re)download is free.

If you want to make purchases or update / (re)download apps, including free apps, you need to update your payment method or add a new one. 

• If you want to change your card, add first the new card, then remove the old one.
• or if you use your Apple ID balance, set your Apple ID balance as your default payment.

See also -> Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

You can also use your Apple ID balance as payment method: Add money to your Apple ID balance

Add money to your Apple ID balance – Apple Support (UK)

What you can buy with your Apple Gift Card or Apple ID balance

What you can buy with your Apple Gift Card or Apple Account balance – Apple Support (UK)