When your training for endurance and you're going out for really long runs (4 hrs plus), eating whilst exercising is really hard work on your body and can result in digestive discomfort (putting it mildly!)
That’s why sports drinks are designed to be easily absorbed and therefore your able to get the nutrients a lot faster than by having to digest them.
People feel obliged to feed their bodies while training, but you don’t need to while running. If you feel the need to eat, choose something that will be easy to digest and more importantly test it out while training before using it in the race.