Why do we use anaerobic energy?

First, "energy" doesn't mean anything. Energy is not a physical unit. It is a philosophical concept and an accounting technique used to analyze mechanical and chemical exchanges. In nuclear physics it is defined as a wavelength of light, in biology it is a synonym for metabolism, and in public utilities they say energy when they mean connectivity. Anybody who uses the word in any other context either does not know what he is talking about, or is peddling something you don't need.

Second, "anaerobic" is a code word with a lot of meanings. There is no answer until you explain what you are talking about.