You get a price break and the store tracks your buying habits.
But now there is an amazing new use: the Centers for Disease Control used grocery store club cards to trace the source of salmonella poisoning.
Approximately 244 people got sick and the CDC tracked their purchases through their club cards. The poisoning was quickly traced to the pepper used on the salami at a factory in Rhode Island.
It may not be the last time club cards are used for this kind of detective work.