Because it indicates "this student has a good grasp of subject X".
If everyone at Harvard exits with a good grasp of Calculus, that's useful information. If only half of people from BU graduate with a good grasp of Calculus, that's also good information. BU and Harvard don't need to have the same proportions of A students for it to be a useful data point.
I think the root problem here is that some people see grades as an absolute measurement of whether someone has mastered the material, and other people see grades as a way to rank students relative to each other. If you believe the first, then it should be possible and expected for a class to be able to get all A's. If you believe the second, then it makes sense to widen the dynamic range such that you get a clear rank ordering.
If everyone at Harvard exits with a good grasp of Calculus, that's useful information. If only half of people from BU graduate with a good grasp of Calculus, that's also good information. BU and Harvard don't need to have the same proportions of A students for it to be a useful data point.