We have students who audit a course. We don't want them and the teacher to know they don't have to do the assignment, but if a student chooses to do it, they can.
Excusing assignments shows the teacher that they don't have to be done, but the student can't do it if they want to.
If the grade is left blank, it looks like they didn't do the assignment (and consequently get the penalty for missing the assignment).
It would be awesome if students could do excused assignments. Then they could show as "Ex" so the teacher and yet students could do them if they wanted.