I do not believe it is. I believe it is more for seeing if you have the permission of an adult (or ARE an adult) in order to buy music. I don't buy stuff using the credit card attached to my iTunes account, I purchase the gift cards separately.
You can. I have an account that isn't linked to a credit/debit card. I find that better because I can't just buy song after song and not notice my money disappearing. I go to the store and buy gift cards for myself to buy music. xd