So one the first Castlavania gamesfor the Game Boy Advance (GBA) was this neat little metroidvania called Circle of the Moon. Its main gimmick was that you had a card system of combining two cards to create whole new effects, like creating a flaming whip, powering up your sub weapons, etc. The problem was that the drop right for some of the cards was INSANELY low making it frustrating to find them all. (I think the idea was you wouldn't get all of them naturally so it kind of switched up each play through).
Code Mode completely fixes that and gives the player cards at fixed location....including the cards need to get the true good ending. Removes all the grinding out of the game and give the player the full experience of messing with all the cards and their powers. You still have to search for the more rare cards but no more stupid pointless grinding for a less then 1% drop.