system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does a middle octane gas exist? I understand using low octane for price and high octane for quality, but why use middle?

Is it just there for show?  What proportion of a gas station's gas is low, middle or high?
gpt
The thing about different grades of gas, is you should only use the one your car requires.  Believe it or not, 87 will perform better than 93 in a car that's tuned for 87.  

With that being said, some cars require mid-grade.  The ignition timing was increased, but not to the point where the owner would have to get the most expensive gas.  You'll find this in some domestic luxury vehicles.