So, there are pain nerves, right? Different from the movement nerves. Normally you want to have functional pain nerves so they can alert your brain to an injury. If someone has, say, arthritis, or a bad joint or bone injury 30 years ago that has long since healed, those nerves are just sounding the alarm 24/7 even though there’s nothing to be done about it. So, since that pain isn’t giving you any new information you can use to help take care of your body, and is actively impeding your life, why not just remove the pain nerves, or cut them off from the nervous system? They are useless at that point for actually reporting things that would impede the mechanical function of that area, so getting rid of them would be a net benefit.