Credit and other cards can be read by electronic devices. Placing your card in the card sleeve makes your card unreadable by these devices.
If your credit or ATM card has the symbol shown in the the picture circled in red then the card has an RFID (Radio Frequency Identification Device) inside it. Click on the picture to enlarge .
The card sleeve is metalized inside and will block radio frequency signals from reaching or being emitted by your card.