I know for sure ethanol is used to raise the octane in the states of Colorado, Nebraska, Kansas, Iowa, South Dakota, Nevada, Minnesota, ( except super premium which can only be used for off-highway use, such as snowmobiles, and boats ).

I suspect it is also used for this in Illinois, and Indiana, but I don't visit those areas. I visit all the others above frequently.

With the 110 octane rating, and costs of well under $2 per U.S. gallon, it is the cheapest booster for the fuel blenders/distributers.

Some states mandate ethanol must be in all blends.
Some mid-west states ( NE, IA, KS ) do not put it in the lowest grade.
And in Minnesota, the ethanol capitol of the world, I cannot even find 87 octane anymore... 89 is the lowest sold. My car gets a jolt every time I'm in CO and UT when I fill up with 85 octane. 85 always boosts my MPG. But you have to be over 3500 feet elevation to find/buy 85 octane.