Why do you want to sign the card?
For the safety of the funds on your card. When you swipe your card, you usually have to sign it. For example, when swiping a card after shopping at a POS machine, it is generally checked whether the signature is the same as the signature on the back of the card. Also, non-molecules often exchange packages with ATM machines. If you sign it, you will find that your card has been changed at a glance.