Coin
"Gives you two extra coins. The more coins you have, the faster your vehicle will go."
~ Mario Kart 8 Deluxe Info PageUse
When used, the player throws the coin up in the air, and it lands back on them. The player's coin count is increased by +2, up to the limit of 10. If the player has 10 coins already, using this item effectively does nothing.
Speed Increase
While the coin item may seem useless at first, it's actually very important.
For one, coins increase your speed. Each coin in the player's coin count increases your speed by 0.05 units per frame.
With 10 coins, your speed is increased by 0.5 units per frame. This applies to ground, water, and anti-gravity speed, but air speed is not affected.
This equates to about a 6% speed increase with 10 coins.
To be precise, a 6.689% increase with a minimum speed stat, or a 6.2893% increase with the maximum speed stat.
Divide by 10 for the speed increase per coin.
The Bullet Bill item's speed is also affected by the player's coin count.
Strategy
While the coin item will increase your speed, if you are in first place it's usually best to hold on to it rather than using it right away. Holding a coin in 1st provides two main benefits.
1. It protects you from getting another coin.
Since you can only hold one coin at a time, you can keep a coin in your first item slot with an empty second item slot.
When you open an item box, you are now guaranteed to recieve an item that can be used for defense against red shells.
Since there is a 69.7% chance of pulling a coin in first place (after 30 seconds), it is likely that you will be left defenseless if you hit an item box with no items.
2. It protects you from the Boo.
The Boo steals an item from a random player in front of the player who activated it. This means that in first place, you have the statistical highest chance of your first item slot getting stolen. This can totally ruin your race by leaving you defenseless against red shells. By holding a coin in your first item slot, you ensure that your defense item is safe from being stolen. If you see a red shell or other threat coming, you can quickly use the coin and begin trailing your defense item.