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Question: This question applies to all Mario Kart games except Mario Kart 8 and leaving out that they're part of the Mario universe. What is the purpose of collecting gold coins? They can't be used to buy anything and even if a player doesn't have any and falls into a pit, Lakitu will still help the player out.
Answer: Across all games, collecting at least 10 coins gives a slight speed boost. In some games, collecting coins unlocks things.
Answer: Absolutely nothing.