Final answer:
Cells carry out fermentation to recycle NADH and allow glycolysis to continue. Although fermentation does not directly produce ATP, it enables ATP production through glycolysis. The production of toxic waste products like ethanol and lactate is a side effect of fermentation.
Step-by-step explanation:
Fermentation is carried out by cells to recycle NADH back into NAD+ so that glycolysis can continue. It does not directly produce ATP, but it allows for the production of ATP through glycolysis. Fermentation produces toxic waste products such as ethanol and lactate, but the main purpose is to regenerate NAD+ and sustain energy production in the absence of oxygen.