One of the ways to boost fuel economy in cars is to use turbo boosting. This way, a smaller engine using less fuel can output the same power as a larger engine. But turbo boosting and higher compression ratios necessitate the use of higher octane fuels. Ethanol can provide that higher octane, thus giving it a value beyond what its BTU content would indicate. Most current flex-fuel vehicles do not take advantage of ethanol's unique properties like higher octane.
Study Finds Automakers to Need Higher Octane Fuels to Meet New Mileage and Emission Standards BioFuels Journal:
At a blending octane rating of 113, ethanol and higher ethanol blends are uniquely poised to help automakers achieve stricter fuel economy and emissions requirements.

While most measure a fuel’s mileage based on British Thermal Units (BTUs), new engine technologies designed to meet higher fuel economy standards like turbo-boosted, downsized engines will require the higher octane level that higher level ethanol blends offer.