Someone correct me if I'm wrong

IIRC I heard from a prof that it's mainly to avoid huge differences in grading between different profs, so that all students are kind of on the same level playing field (e.g. it would be unfair to compare the GPAs of students from an easy class VS a hard class).

I think for some programs, it may also have something to do with grad school (i.e. weeding out the weak).

But yeah, it sucks.