There are a couple of thoughts behind the idea of carb loading:
1. Your body can more quickly process carbohydrates than it can fats and proteins. That is, they are quickly broken down in the stomach, where some simple carbs can be immediately absorbed into the bloodstream, and on into the small intestine, where they continue to be absorbed into the bloodstream and on to the muscles or liver for storage.
2. It takes the body less energy to convert carbohydrates to ATP, which your body uses for fuel. Glycogen is kind of a temporary rest stop in the energy burning cycle, and is the form that is used to store the supply of fuel, both in the muscles as well as in the liver. There are small differences (as I recall) between the structure of muscle glycogen and liver glycogen (which is used to supply the brain with fuel).
3. In general, it is much easier to digest carbohydrates than proteins or fats. I have had experiences where I have eaten a fatty meal at 5 PM, and have gone out for a run at 9 PM, only to still feel the food in my stomach and feel yucky.
Your question below takes all the science out of it and makes it nice and simply - yes, the typical approach is simply to eat pasta (and bread) or other complex carbs that will help you to ensure that your muscles have the maximum glycogen supply for the race, since that is the easiest form of fuel for the muscles to use. Once you run out of your glycogen supply, if you don't replentish with easily digestible carbs (thus gels/blocks and gatorade), your muscles will be forced to utilize other less efficient forms of energy (i.e. converting fat to fuel, which is very energy intensive and slower than the process of converting carbs to fuel). Long-winded answer for a simple question . . .