If you remember back to Animal Crossing on the GameCube the normal NES games that you could obtain each came as an individual furniture piece that appeared as an NES console with a single game box on top of it. When you placed the item in your house and interacted with it, it would only play that one game.
It was also possible to also obtain a generic “NES Console” item that did not feature any of the built-in games. You could buy this item from Redd, or sometimes obtain it through random events such as town bulletin-board message.
It turns out this generic NES Console can load NES roms from a GameCube memory card as James Chambers explains in his blog.
Nintendo might well have planned to distribute additional NES games on memory cards at some point, however decided against that. So its pretty neat to see unused features enabled on GameCube games all these years later.
You can read the very detailed article on James Chambers's Blog.