Why is there 3 wire system to 2 wire system?

I assume you mean hot, neutral and ground. The answer is simple...the advantage is life or death in a home wiring situation. The only true job of electricity is to seek ground and it will find it through the path of least resistance. If the neutral wire of a 110v 2 wire system becomes open, the only path to ground may be your body. If that same wire opens on a 3 wire grounding system, there is a backup path through the ground wire creating a low resistance to ground. Safety issue.