The anaerobic cellular systems or anaerobic respiration use fermentation because the anaerobic cellular systems oxygen is absent. Fermentation takes place in the absence or in the partial pressure of oxygen to produce alcohol or lactic acid. This is the way of production of ATP which is need for various cellular processes.
If fermentation was not there, then the anaerobic cellular systems could not produce energy and the cellular processes could not occur. In the absence of fermentation, no ATP would be produced and the cells would gradually die.
