Because they can evaluate where you are and determine if there is any gap to where you should be. That way they can manage your expectations and where you should concentrate efforts.
It’s called managing. It is helpful to those being managed.
So now you are a fifth year senior and it takes you 40 hours to make a single slide in the deck. Guess what, if your professor isn’t telling to you may not cut it, he/she is a fraud. Managing.
2 third year students have to make 10 concepts in a week. Student A does the 10, student B does 15. Both receive an A. Do you honestly think student B should get an AAAAA++++++? Managing.
Again, there is little, if any, difference between manager/professor. The manager’s/professor’s expectations of their associate/student should fit an experience level. If an employee hits that expectation, they get paid. If a student hits that expectation, they get their grade.