If this is a scammy casino, well, maybe they want to sell your credit card details to hackers who will try to steal your money. If this is a reliable and reputable casino recommended to you by respectable reviewers, then they may request your credit card details to check if you are actually 18 years old, and verify your personality to make sure you do not claim their no deposit bonus from two or more different accounts. Thirdly, if their bonus is cashable, they will need to send your gains somewhere - perhaps on your card.