I have an itunes account on my Iphone 8 but when i try to download free apps from app store it asks for verification. Why for free apps?

There are normally some hoops you can jump through as you create your iTunes account that workaround the need for a payment card or bank account, otherwise once the account is set up the iTunes Store may still expect you to provide one, even if you are obtaining free content, which you can later remove. If you don't have or want to use a credit/debit card at this time you can try this method using iTunes on a computer...

1. Enable the menu bar if needed with Ctrl+B (only applies to some builds of iTunes 12 for Windows)
2. Sign out of the iTunes Store from the Store/Account item on the menu bar
3. Go to Music > iTunes Store home page
4. Go to Quick Links > Redeem
5. Sign in with your Apple ID
6. Go through the screens, accepting the EULA, etc. You should arrive at a Review Payment screen where one of the options is None - select this and continue. You shouldn't actually need to redeem a new voucher/card.

It is worth noting that one of the functions of the registration is to prove you have a valid billing address in the region whose store you are trying to use. You might also have difficulty selecting None as an option if you have any active subscriptions (see View, change, or cancel your subscriptions - Apple Support), an outstanding debit balance to the iTunes Store, there are unreleased authorization holds on the card (see About payment card authorization holds in the iTunes Store - Apple Support) or you are trying to switch to one regional store while being in a different region. See If you can't choose None when you edit your Apple ID payment information - Apple Support for more.

tt2