Simply including coins as collectables encourages players to collect them. Their placement at regular intervals provides a constant short-term goal, which lends itself to flow. Their layout is designed to highlight the curve of the track, which is especially useful in the latest games in the series, which tend to have a lot visual distraction. You’ll find that wherever the track splits into two or more different routes, coins are always placed at their entrances, making it easy for players to pick a side early.
 

The average total number of coins held by a racer increases over a single race, which raises the average speed and the tension as the race progresses. Combined with the difficulty ordering of tracks in a cup and the natural tension which emerges from the tournament system, we end up with a typical pacing curve. Tension increases during a lap, decreases slightly at the start of each lap, and decreases a little more at the end of a race.

 

 The coin item increases the spread of item value. Where the banana used to be the least valuable item, and was very effective at holding behind your kart in order to stay safe from attacks, the coin now is; meaning that players in high ranks are likely to receive a coin rather than a banana. This makes the players at the head of the pack a little less stable than they had been in past iterations of the series (although the inverted question box once had a similar role), and the banana feels more valuable in the same as a result.
 

Rewards (characters, karts, stickers) are presented based on the total number of coins collected in the game. This will be roughly equivalent to time spent playing the game, which allows designers to ensure that a player receives any reward at a particular time; but because it’s an active metric rather than a passive one, rewards feel more deserved.
 

Coins are lost when a player falls of the map or is injured. A common complaint with Mario Kart as a series is that an expert player can lose to a novice due to the rubber-banding provided by the AI and the incredibly powerful items handed to players at the back of the pack. As well as simply making the items and their spread more contest-orientated in Mario Kart 8 (which is another article for another time), the coins divide players based on skill. Better players will find themselves losing less coins on average, making up for worse items with a higher top speed; making it easier for them to get ahead of the pack and concentrate more on racing. The game rewards consistent racing skill as a means of staying ahead. Less experienced players will tend to have less coins, but better items, allowing them to experience the more traditional Mario Kart experience, leveraging items to more than racing skill to get ahead. In this sense, you could argue that simply learning about what coins actually do is a key distinction between these two types of player, and that hiding the complexities of the system to entry level players makes it easier for them to simply enjoy the game.