Since it is your personal and individual scale of pain the doctor knows how much pain it is for you. That is important because not his scale is important to predict when you cannot bear it any longer. In terms of diagnosis it is also helpful to know if you are the type for feeling pain early or not. it is not about absolute accuracy but about the level of pain load you can or cannot bear. When a patient faints due to pain it isn't of importance that you would sense the same pain level as nothing. As a doc you will want to prevent a fainting person to hit the floor with the head, you know?