What is the importance of oxygen for facultative organisms?

Facultative organisms have the unique ability to survive in the presence and absence of oxygen. Although oxygen is not absolutely essential for facultative organisms to survive, oxygen is important because these organisms grow more effectively in its presence. 

When oxygen is available, facultative organisms switch over to aerobic respiration, which produces much more ATP (adenosine triphosphate) as compared to anaerobic respiration. ATP is the primary usable source of energy for cells, so more efficient ATP production is a considerable growth advantage.