Well its just another source of energy...
Aerobic metabolism (TCA) is primarily used when O2 is present and Anaerobic metabolism (glycolysis) is primarliy used when O2 conditions are low, such as exercise. Why waste the NADH? You need glycolysis to make Acetyl-CoA anyway.
NON MCAT MATERIAL TO FOLLOW
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One use of the Anaerobic energy production system is that, while the TCA produces MORE ATP per glucose, Glycolysis can produce it alot faster. In exercise, energy consumption very quickly saturates the speed at which the TCA can churn out ATP. Instead the Anaerobic system (glycolysis) steps in and can produce ATP much faster. This produces Lactate, because the pyruvate that is made by glycolysis must be fermented to create NAD+. This NAD+, remember, is required for Glycolysis to continue. So during exercise, glycolysis is self-propagating. It uses NAD+, makes NADH and ATP, and then through fermentation to lactate, it regenerates its NAD+ so glycolysis can continue. This fermentation to lactate is referred to as the 'oxygen debt'. This is because when O2 becomes present, the lactate can be reconverted to pyruvate, and then be used in the TCA. The lactate buildup is why you get sore muscles.
There is also the Creatine-Phosphate ATP buffer system to give you ATP for the first few seconds of exercising but I wouldn't worry about that.