The objective of traditional grading is to compress information teachers have gathered about a student down into a single score to make understanding the information easier. One of the original reasons for this compression was the limitation on how much information could be shared on a single piece of paper. One of the purposes of comments is to uncompress the grade a bit, so that parents and students have some ideas on how to improve their grades.
This process is used to change the size and quality of pictures as well. Compare the two pictures below, and ask yourself, which one conveys more information?
Is there a way we can share information parents and students can understand, while not reducing the information too much?