Unlike the E85 you can get at pumps in many locations now, race fuel companies are offering higher octane blends of E85. At the pump they’re essentially denaturing the ethanol with horrible quality gas to bring the octane rating down around 100-105 RON.
By mixing it with unleaded race fuel some companies are now offering 115 RON E85. It’s higher octane, has better burn characteristics since it’s being blended with race gas, is still safe for o2 sensors, and the blend will be FAR more consistant.
When I ran E85 from the pump I had to adjust my fueling every time I fille up because the blend was always a little different.