One key advantage is that anaerobic respiration occurs without the presence of oxygen, in contrast to aerobic respiration, which requires oxygen. Another advantage is that it releases energy rapidly, making it useful in times when the process is necessary for the cell, such as following heavy exercise. An additional advantage is that its waste products (gasses) can be used as renewable energy sources. For example, biomass decays and generates methane as a byproduct, which is a significant energy source.