there's a limit to how much carbohydrate you can aborb, generally somewhere in the range 60-100grams per hour. or about 240-400 calories. so you can't take enough carbs to fuel your race fully. the shortfall has to be made up from stored glycogen, and fat burning. glycogen capacity is something like 1800-2000 calories, so that runs out quickly, and the balance has to come from fat metabolisation. over an 140 mile race, being able to burn 500 cals/hour of fat rather than 250 cals/hour can make a huge difference.