What are coins for in Super Mario Kart on SNES?

In the oldschool super mario kart for snes you could collect coins that lie around on the track.

What do they do? Why should I collect them?

Back in the days as a kid I would collect them because they are a shiny. I doubt that is everything to it.

Solution 1:

Each coin held slightly increases your top speed on the road, by approximately 0.8%-1% per coin (the exact percentage depends on the selected driver, the kart class, and whether you're playing a PAL or NTSC version). For example, a Koopa Troopa with 10 coins will have the same top speed as Bowser with 0 coins.

As michaelb958 notes, you only need 10 coins for the maximum speed bonus. Any additional coins will not provide further benefits.

You will lose 4 coins if are hit by an item that causes you to spin out, 2 coins if Lakitu rescues you, and 1 coin if you hit another player. If you crash into another driver while you have 0 coins, your kart will spin out.

Coins do not affect any other aspects, so drivers are still going to vary a lot in acceleration, handling, etc.

Solution 2:

Also, when a player-controlled driver is hit, they lose a coin. If a player-controlled driver is hit with zero coins, they will spin-out.