There's no dietary or metabolic need for carbohydrate. The human body can survive and thrive perfectly well with only fat and protein as macronutrients. Practically speaking, doing that is challenging and complicated because most foods are a mix of all three macronutrients.
But do you need sugar? Or you don't need either sugar nor carbs? It seems strange to me.
EDIT: it seems that if you use only your fat, you'll end up producing too much Ketones which leads to Ketoacidosis. Doesn't sound good.
I think the reasoning goes because you are used to using carbs as your energy source most of the time, your body doesn't know what to do at first when it doesn't have any available. The body adapts surprisingly well to different energy sources, so if you drop all carbs, your body will burn fat for energy and create ketones.
It kinda sounds crazy, and I used to think it was, but I'm currently low carb to lose weight (lost 28 pounds in 3 months with it so far!). It just takes a couple weeks of training while on the diet to get your body to catch on and be back to your usual performance.
There are actually a surprising number of world records set by athletes on low carb diets who destroyed previous records that claim their diet is the main reason they were able to accomplish their goals