The point becomes clear when you look at it from the teacher's / professor's point of view. We want to read and provide feedback for YOUR work that will help you develop the skills you need.
If you went to a medical doctor, would you hold the thermometer against a light bulb, or hold your breath, or try otherwise to give him or her an incorrect reading on measurements of your heart rate? No, of course not.
Similarly, we want to evaluate your work for you, as you are able to produce it. We feel it's a waste of time marking and giving you credit for work that's not actually what you can produce. Why would I want to give comments to you on someone else's words?
So, it helps you most when you don't rely on any aids, or, if you do, if you let us know where you have done so. Then we can keep the feedback appropriate and relevant.
Student or Learner