Why use them?
Because carbohydrate is stored in both the muscles and the blood, your performance on race day relies on using the glycogen stored in the muscle. For glycogen to make its way to the muscles, it must first be digested, make it’s way through the intestinal wall, and then absorbed by the muscles. This process takes time and isn’t very efficient.
However, gels will often “wake you up” in a very noticeable way because our brain only runs on the glucose stored in the blood. As the muscles start to absorb more blood glucose, the brain gets less glucose and starts to get hazy (you’ve probably noticed this feeling on your long runs, or if you’ve gone for a run without eating enough). Often, a gel will wake you up and help the mind feel energized, but it doesn’t necessarily prevent the bonk in your legs.
In summary, energy gels help replenish the glycogen and calories you’re burning when racing hard. However, they aren’t very efficient or a simple one-to-one replacement, so timing and frequency are critical factors to avoiding the bonk.