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!