Why do I have to write so many drafts of one essay?
Ralph Waldo Emerson said, "Life is not a journey, not a destination." In writing, the goal is not to reach the destination (the finished paper) as quickly as possible. The goal is to take the journey with multiple drafts. Your teacher/professor wants to see how your thinking about a certain topic develops over time and how you incorporate his/her comments into the next draft. This is not possible with just one draft.
I confuse the simple past tense wth the present perfect tense. Aren't they both used to talk about the past?
Yes, both grammar structures refer to the past. The key is to remember that in the simple past tense, the action started in the past and finished in the past. The three functions of present perfect are: 1. To talk about an action that started in the past but is still happening now, in the present. For example, "I have lived in NY for 23 years" uses the present perfect. This person started living in NY 23 years ago and still lives in NY now. 2. To talk about an action that happened in the past at an unknown time. For example, "She has moved to California" uses the present perfect. We don't know exactly when she moved, only that it happened. 3. To talk about a repeated action in the past. For example, "I have seen that movie three times" uses the present perfect. The repeated action in this sentence is seeing the movie.
Why do I need to write essays in classes that are not English classes?
Writing is a way to organize, structure and articulate your thought processes (about any topic). This is important in non-English classes like math or science because when you arrive at an answer for a problem, you have gone through a thought process. Analyzing your thought processes through writing is just as valuable as getting the right answer, and extremely helpful when you come to an incorrect answer.