The point is to train you to be a scholar, someone who can do original research in the field, as is expected of university professors. To that end, a PhD program includes coursework advanced enough to bring you up to the current state of the research in the subject, and a period of research apprenticeship with your advisor, where you begin to contribute to the field yourself. That's valuable enough to devote four years to if scholar is the job you want. I wouldn't recommend it for any other purpose. Way too much pain.