To answer your question simply it's to steal other's energy. I believe that every person has this energy inside of them that sustains them, cuddles them, gives them life. And energy is what we as humans crave. Have you ever met a person that you were intimidated by, and you didn't know exactly why? It was because they were stealing your energy. Until humans learn to use the energy they have bottled up inside they will try to take it through an outside source. When we feed off others energy we need more and more to sustain ourselves. This causes wars, conflicts, murder, rape, genocide, hate, and other negatives. There isn't any kind of science out there that would be willing to test my theory, and to be honest I don't know if we have the science to test it. All I can say is that when we as humans evolve to using our own energy we won't need to steal it from others.