Some online casinos will request card details during the registration process, as they use this information as part of their KYC process.
This process ensures that the casino has verified the identity of its players, promoting a secure and responsible gaming environment by preventing illegal activities like fraud and money laundering.
By collecting essential information at the outset, casinos can uphold regulatory standards and maintain a transparent relationship with their players. That being said, this is probably the lesser reason a casino gathers your information at this early stage.