U of Michigan Ann Arbor LSA or Engineering

I know this question seems a little odd, but does the math subscore really matter If I am in Calc 3 and will complete Diff Eq before graduating. I just don’t understand how a predicted act score for a college course is more substantial than already completing a good amount of college level courses in that section.

I know the ACT is important, but it is an academic predictor. If I have already completed the courses upon which the ACT is predicting my success level at, is the ACT really needed at that point. I mean is the ACT more important than the actual college result?