The Nintendo 64 Expansion Pak

Many fans of the Nintendo 64 will likely remember the Expansion Pak (I swear that's how it's spelled- leaving out letters in words was the cool, rebellious thing to do back in the 90s) , a peripheral which increased the system's RAM, enabling more detailed graphics.

The first major game to utilize the upgrade was Rogue Squadron. The use of the peripheral was strictly optional however, and the game was just as enjoyable (though slightly more jaggity and and foggy) without the Expansion Pak.

The first Nintendo game to require the Pak was Donkey Kong 64. Nintendo, being the nice charitable (read: shrewd) organization that they are, shipped the Pak with each copy of the game, to help players avoid any extra cost.

Nintendo

Nintendo

The title requires the Pak not because the game world was too big, or because the graphics were too detailed, as was the case with Majora's Mask. Nope, the simple fact of the matter was that the game kept crashing until the developers crammed the Expansion Pak in their test consoles. They never did figure out what was causing the game to crash so Nintendo bit the bullet and decided to ship the upgrade with each game. 

Source: didyouknowgaming.com