Why do graduate programs have comprehensive examinations (comps)? I'm about to begin studying for the comprehensive examinations (comps) my masters program (Computer Information Systems) requires.  I'm curious as to the point of even having them in the first place.  I'm not concerned about whether or not I'll pass my comps.  I know that I will.  It just seems like a lot of wasted time in both preparation and the fact that I'll have to burn a precious day of vacation to take them.

I could understand if, as stated in another post regarding PhD programs, we were required to take them after our first year of the program as part of a larger weeding out process.  However, that isn't the case.  We're simply required to take them before graduation.

In four separate exams, we're tested on the four core courses for our program.  Didn't I pass those courses already?  Haven't I already earned a satisfactory/passing grade for each course?

Would someone please provide my ignornant mind with some insight into the possible reasons why?  It feels like double jeopardy...  Thanks!

EDIT:
The exams are written by the same instructors who taugh the specific class I took.  For example, both Dr. Jones and Dr. Smith teach CourseX.  However, since I took Dr. Jones for that course, he is the person who wrote the comp for the course.  At least, that's my understanding of how they're written and by whom.  I apologize for leaving this nugget out.  I shouldn't write these questions while I'm sleep deprived...
The question is about exams taken at the end of a master's program. I have given such exams. In my mind, they make the candidate prove that they have mastered the material of the courses, which is a form of quality control. But the exams also have a few secondary goals in my mind:


By studying for the exams, students gain more understanding of the material, and come away better educated. The actual taking of the exam is less important, in a sense, than the time spent studying. If a student studies well, they should pass the exam. 
The exams keep the faculty honest when teaching, because the faculty know their students will have to pass exams that will be seen by other faculty. Exams within a course are often not seen by other faculty, so the instructor is not as publicly accountable. 
The exams help the faculty see what the students know, all in one setting. We can use this to revise courses at the departmental level if we find that many students have the same gaps in their knowledge.