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 third and final variation of the Nintendo 64 Controller Pak, and it came in blister packaging. The first two variations came in cardboard boxes. The part number above the UPC is NUS A PA2. whereas the two cardboard releases had the part number NUS A PA. The part number on the blister package is U/NUS-A-PA2-USA. The Controller Pak did not change its model number (NUS-004) during the Nintendo 64's lifespan.