When carbohydrates are absorbed by your body, they are broken down and converted into something called 'glycogen'. This glycogen is the primary fuel source for your body, both while exercising and leading your average lifestyle. Glycogen is very accessible to your body - it's easily converted into energy. In comparison, fats and protein both require more energy for the energy in them to be released - think of it like paying tax on your body's fuel! Fats are generally burned during long duration activities that require little energy. This is why members of the Atkins diet find it difficult to do anything really exhilarating. At the same time, if you use all your protein for energy, you can't use it to repair your muscles. This is the key answer as to 'why do we need carbohydrates'.