Fully loaded glycogen stores in the body make for higher quality workouts and better performances in competition. This has been common knowledge for over 50 years. But recently a more periodised approach to nutrition, often summarised as “train low, compete high”, has begun to gain favour among endurance athletes.
In essence it means varying the volume of carbs you consume during given periods of the training program in order to trigger metabolic adaptation and increase the bodies efficiency at using fat for energy.