The library card also serves as a method of identification. When a person chooses an item to borrow and presents their library card to the library, they take responsibility for the borrowed item and promise to abide by certain rules, usually including a promise to return the item by the due date or face a library fine.
The library card is used as proof of identity. When someone chooses an item to borrow and shows their library card at the library, they are responsible for that item and must follow the library's rules, such as returning it on time or facing a penalty.