the https://en.wikipedia.org/wiki/Nintendo_64_accessories#Expans... raised main memory from 4mb to 8mb.
it was required for one of the Zelda games, but also helped with graphics for some games if it was inserted.
seemed to be a decent success.
https://www.reddit.com/r/n64/comments/ft63zn/dk64_memory_lea...
The lead artist claims they’d always been planning to make use of it. There’s compelling evidence of a memory leak or similar issue since the released version of the game apparently crashes if you leave it running for some amount of time over 10 hours (which wasn’t usually a problem in practice except for very long speedruns until it was released on Wii U Virtual Console and people started using save states which don’t reset the timer the same way as turning the game off and on does).
I _think_ it was on the documentary clips shipped with Rare Replay, but I'm not 100% on that.