The Nintendo 64 Controller Pak was used in certain games to save data that could not be stored on the game cartridge itself. It was used to save ghost data in Mario Kart 64, and to save the game itself in Quest 64. Most games published by Nintendo saved their data to the cartridge itself, and many third party games required the Controller Pak to save data.
Trivia
This was the second release for the Nintendo 64 Controller Pak, and it also came in a cardboard box. The part number on the box flap is NUS-A-PA-USA-1. The part number above the UPC is NUS A PA. The Controller Pak did not change its model number (NUS-004) during the Nintendo 64's lifespan.