Carbs - your power and your fuel
Weightlifting and carbohydrates:
To perform a workout or to play a sport, adequate carbohydrate stores are what makes you perform to your best ability and prevents premature fatigue. Carbs are stored as glycogen in muscles or in liver (released to blood as blood glucose ready for use).
One can assume correctly that glycogen stored in the muscle is what makes your muscle work (contract).
Blood glucose on the other hand is mainly used by your nervous system as a fuel, as well as by muscles and other organs. Low blood glucose levels during exercise is what makes you feel fatigue and unable to carry on with exercise. Especially with added dehydration your performance level is lowered. Nervous system can not function properly without proper fuelling, therefore muscle and cognitive functions drop, even if your muscles would still have energy (glycogen) and capacity to carry on and complete a workout.
As carbohydrates push you to train hardest they are the nutrient that allows room for development. This can bee seen for weightlifters wanting to enforce strength and hyperthrophy. In order for your body to develop, it needs to perform stimulus with higher intensity and recover for adaptation to occur.
Recovery requires replenishing carbohydrate stores, which get depleted by training. This can be done in 1-2 days with normal amounts of carbohydrates in your diet. Good window to replenish carbohydrate stores, especially if you want to train more frequently, is in the first two hours after exercising. This is because your body synthesises carbohydrates at a much higher rate post exercise.
Even though carbohydrates are not the macro nutrient to enhance muscle build, it is the only macro nutrient that allows you to train hardest, thus making it possible to develop as you are able work a bit extra and make that extra push.
It is the only macro nutrient that allow your muscles to work hardest and result in muscle gains.
Once again, not one, not two, but all three macro nutrients are required for any healthy lifestyle.