Is it physiological or psychological?
A specific gel or specific energy bar or energy drink or whatever... is it actually affecting the body in a way that will assist the race or is it affecting our mental state? Both are valid reasons to do it.
The body does need fuel.
Most of us have enough fuel in reserve to go for hours and hours and hours. Fuel is not the issue. Intensity is the issue. The level of intensity determines the kind of fuel being used and how. What distance can we go before our body decides we have gone too far too fast? It's not an energy issue; it is a conditioning issue.
What kind of fuel?
Carbs? Carbs are good. They are quickly metabolized for energy and are the preferred source.
Proteins? Proteins are needed to repair tissue, so the body prefers not to use it for energy unless it has to.
Fat? Fat has over twice the energy as carbs. It takes longer to turn to energy, but it gives more.
What about blood sugar?
That's the kicker. A stable blood sugar can be maintained burning body fat, but a system that is used to plentiful sugar via cookies, ice cream, cokes, candies, pastas, starches, fast food, pizza, etc, etc, will crash pretty hard when low on readily available sugar sources. Lethargy in the system is what hurts us. Understanding our bodies is a good way to manage energy.
We can have pain and still be fine. We can have nausea and still be fine. We can have doldrums and still be fine. We have to understand our body's timeline for things. How long before we go without caffeine do we get a headache? How long before we eat do we get a headache or nausea? How long before they go away? How do we train our bodies to be efficient with our energy systems?
I think people who are competing in races have enough information to be smart about their bodies. I think others who come to running or other endurance events without that knowledge are in for a hard time. People die at these events. So, being on the safe side gives people both the extra boost they need to feel good at the race and the peace of mind that they can finish.
Take everything with a grain of salt; keep asking questions.