My current master's class is about measurement and assessment. I admit that this class is far more interesting than I thought it was going to be, in part because the text is exceptional. (It's an online course so there's little interaction with the instructor except via email.)
I'm going to ask a question that motivates the textbook author's thesis. I'd like you to answer the question in the comments, and if you're a teacher/professor, identify yourself as such if you're comfortable doing so. I don't want you to give me what you think is the "correct" answer, or what you think the author would say is the "correct" answer, but instead just answer honestly. I have hopes that this will lead to an interesting conversation. After there have been some responses, I'll post the author's ideal answer (which I kinda like, but wouldn't have been my original answer).
First, a lead-in question for teachers and professors: For what reasons do you assess student learning?
And the biggie: What is the most important reason teachers should assess student learning?