system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What's the difference between the different varieties of gasoline, why do they exist, and who even buys anything other than regular and diesel?

There's usually 4 varieties of gasoline at most gas stations. While I don't know the difference between regular and diesel, I do know that certain vehicles can only run on the latter (but don't know why).
gpt
The engine has to squish the mix of air and gasoline together before it makes it burn with the spark plug.  But some engines reeeeeally squish it a lot (because they're trying to be more powerful), which can make the mixture burn on its own while the engine is still trying to squish it.  This makes the engine break.

So for these engines you have to make the gasoline harder to start burning.  This 'how hard is it to burn' rating is the octane number on the pump, and the higher the number the harder to burn (and the more expensive to make it).  Since new cars try to make the same power out of a smaller engine, a lot of them squish the mixture more by using a turbocharger (basically a fan that pushes in more air), which is why they need the more expensive gasoline.

If your car doesn't require it, higher octane fuel just wastes money.  It won't make your car more powerful.