- Most obviously, it is a painful hurdle along the way to a PhD. Some students do fail, and others quit as they look towards the exams. So, the exams do have some role in selecting who moves on, even if in most departments failure is rare.
- It is diagnostic--not just in the sense of figuring out who should leave the program, but also what folks might need to do to improve their comprehension and address their weaknesses. The oral component, if one your department has one, is good practice for later on when the stakes are higher: job interviews.
- The old saw is mostly correct--you will never have a better view of the entire field than at that moment in time when one is taking the comps. The exam forces one to read beyond one's interests and inclinations and to try to figure out how the various conversations speak to each other.
- The analogy I always use is sport. When you first step on to a field or court, the movement of the players is so very confusing that you do not know anyone's roles and where you fit. The more you play (read), the more you understand why everyone is where they are on the field, where they will be, and where you fit.
- Once you prof-ing, you will not have the time or incentive to be well-read across the field--just in your areas of research and a bit beyond. At that moment in grad school, you have one and only one job (aside from perhaps some RA/TA work): passing the exam. I have enumerated elsewhere the many tasks profs must do--being up in the literature especially in another field is just down the list.
- The process may cause one to learn of literature that might be handy for one's research. I had my dissertation question pretty much in sight when I was preparing for my comparative comp, so I focused part of my reading for that exam on the literature on ethnic conflict, an area that I had previously studied much. It not only helped me develop my answer to my dissertation question but ultimately shaped the next ten years or so of my research. While that is just my experience, I do think think that I have observed students growing a bit and moving beyond their previous stances because the comps pushed them to read stuff that they would have otherwise skipped.
* Much of this depends on one's department. I had a great set of colleagues in grad school, and we helped each other as we went through it. In other places, where the students see each other as competitors rather than collaborators, the exams are particularly painful.