Obviously not every run would be fasted especially if you don't always run in the morning. Makes sense though that conditioning your body to perform under glycogen-depleted conditions routinely would increase fat burning utilization/efficiency. This would translate into being able to hold marathon pace while burning a lower percentage of carbohydrates than if you were not doing fasted runs. This means you would require less fueling during the race and would be less likely to hit the wall.
People like to say, "It's just a mental thing" or that increasing weekly mileage is enough. I don't think it simply comes down to an arbitrary number of weekly miles. It's how you train that is key. Train specific to the race you are doing. Having really good fitness doesn't mean squat if you always bonk at mile 18-20.