A couple of years after I started driving I thought 89 and 91 octane must be better because it costs more. I didn't want to make the jump all the way to premium so I'd get mid-grade to treat my car to better fuel. The placebo affect alone felt like an extra 10hp, combine that with a car wash and a new decal and I had a real sport's car on my hands. I would say mid-grade's primary reason to exist is to capitalize on the clueless. I read all the time on other car forums about people using mid-grade or premium when they don't need to because it makes their car feel more peppy and smoother running. I suppose a few extra bucks a month to feel better isn't a bad deal.