The Nintendo 64 Expansion Pak was used in conjunction with certain games to allow for enhanced gameplay. Donkey Kong 64 and The Legend of Zelda: Majora's Mask required the Expansion Pak for gameplay, whereas games such as Hydro Thunder and Perfect Dark used it to unlock multiplayer and other features. Re-Volt, Road Rash 64, and other games used the Expansion Pak to unlock higher resolutions.
Trivia
The initial release of the Nintendo 64 Expansion Pak came in cardboard packaging. The part number above the UPC is NUS A TB. The part number on the box flap is NUS-A-TB-USA. The Expansion Pak did not change its model number (NUS-007) during the Nintendo 64's lifespan.