It's a metabolic intermediate of the citric acid cycle - the cycle is needed to reduce electron carriers (NAD+/FAD) which are then used to drive the electron transport chain for making ATP. In simplest terms: it's an important part of aerobic respiration.

That being said, it can be synthesized de novo very easily so long as your cells have enough glucose/fatty acids and enzymes are functioning properly. It's even formed directly by deamination of glutamate. I'm not sure these results would be very transferrable to humans.