Why does iTunes sometimes ask me to verify my account?

Question

Fintech

Hello everyone
From time to time, when I try to download something or buy something in the AppStore or tuna, I am asked to verify my account. To do this, you need to enter CVV, and then they take a dollar from me. Everything would be fine, but it began to happen often. Once every two or three weeks. Why is this done and is it normal?
Thanks