I can't speak as intelligently as some may on this particular topic, BUT, if one is eating @ 40 g carbs daily, that's enough to fuel workouts (along with ketones).
Additionally, carbs are of more use when trying to build muscle, and on a contest diet, my goal is to maintain muscle while losing as much bodyfat as possible.
The high protein spares muscle be preventing muscle breakdown for amino acids and the fats provide energy as well as helping keep the hormonal profile as advantageous as possible for keping muscle.
Again, I'm no expert, but this is my understnding and it worked wonders for me.
One other thing, with such low carb levels, insulin surges and dips don't become an issue, so I felt the best I ever have on a diet. Not grumpy, not overly fatigued and looked my best.