why is doctor's work know as practice?

does this mean to say that you are not aware that once we put our profession at work, it is called a practice of our profession? since i am a civil engineer, if i go into jobs wherein my knowledge and skill of it is put to use, i am a practicing civil engineer. those acive accountants are called practicing accountants and all those active lawyers we see in courts are all practicing lawyers. no, they are not toying with their professions, they are making their professtions get to work. i don't know though why this expression has come to be about the practice of one's profession at work.