It's been a long while since I set up my iTunes account. When I first set it up I put my card number in there, so it would bill me appropriately for what I buy. I'm assuming they just want that info in there in case you do try to buy something? 😕
If you feel unsafe putting your info in there, I've had an account with them for a while and have never had any problems at all with being billed for anything that I didn't agree to buy. 😉