teaching - Why do educators use curve to adjust the performance?

It bothers me when I tries to remember how many highschools or universities use curve to adjust the final grade for their student based on the class average.

I know failing the entire class would be unacceptable for the faculty that taught the class but watering down the materials or curving the grades to fit the standard was it necessary? What if there are few outstanding students that are ahead of the curve? wouldn't it be unfair to them if the curve was adjusted to the class average?

Sometimes, I started to question education standard for grading. What are the standard?

Knowing the materials and able to apply them?

Putting all relevant summary on one test paper?

Giving homework that count towards grade?

Quizzes to remind student how well they do for that subject?

If above are all taken into consideration. Why are instructors still applying the curve to adjust the class average.

Answer

Instructors often use some kind of curve to adjust for the varying difficulty of exams from one year to the next.

The general idea is that if (for example) the class average on an exam is lower than it has been for the same course in previous years but the quality of the students' work is the same, then the exam given this year might have been more difficult than the exam in previous years. It would be unfair to students (and also reduces the signaling power of grades) if their grade is strongly dependent on the year in which they happened to take the course. An instructor might choose to adjust the students' grades to account for this.

It's generally unwise to assign grades in a way that is strictly norm-referenced, without taking into account the students' demonstrated mastery of the course material, for reasons described in this answer. For example, if the class average on an exam is lower than it has been in previous years, but the average quality of work submitted by students is also worse than usual, then the average grade of the class should be lower (to preserve "fairness" and also the signaling power of grades.)

Some instructors might use a curve for other reasons, e.g. if their department has a policy about the maximum number of "A" grades an instructor can give out. (Like this example.)