Ok here we go, Carbs are an important fuel because they are required for ANAEROBIC(short,intense, high velocity/force activites) energy. They are also needed to stimulate INSULIN which is an anabolic hormone. Carbs also keep the muscles full with water(make sure you got enough salt to help do that too). Like the other dude said to they produce and replenish GLYCOGEN(stored form of carbos) stores in the MUSCLE and LIVER(for back up).