Unlike the real world, coins tend to have super natural abilities in video games, such as healing damage or awarding extra lives when a specific amount has been collected.
Coins are frequently found in the Mario series.
In the Mario platformers, coins are usually found floating in levels and hidden in blocks. They not usually used as currency. Instead, when a player collects 100 coins, the player is awarded an extra life and the coin counter resets to 0.
In the Mario RPGs, coins are used as a currency that can be spent on various goods. Unlike in the platformers coins do not award an "extra life", as lives do not exist in these games.
In the Mario Kart series, coins are used to increase your top speed or to unlock vehicle upgrades.