The transcript already reports the grades and the activities list already reports the activities. An essay that repeats them uses the one part of the file where something else could have been said.
The essay is not a list of achievements
Admissions readers have the rest of the file in front of them. What they do not have is any sense of how the applicant thinks.
The strongest essays are usually about something small and specific. A large subject handled in six hundred words produces generalities, and generalities are indistinguishable between applicants.
The subject choice is half the outcome
A well written essay on the wrong subject cannot be rescued by editing. The test worth applying before writing is whether anyone else could have written this piece with a few details changed.
If they could, the subject is not doing its job, and finding that out before the first draft saves the drafts that follow.
The voice has to be the student's own
The essay is read alongside the rest of the file, including teacher letters and, at some institutions, an interview. A voice that does not match the rest is noticed.
This is why an essay that has passed through an adult's hands often reads worse than a plainer one written by the student. The vocabulary rises, the sentences smooth out, and the person disappears.
Using examples
Reading published examples is useful for seeing structure and harmful for choosing a subject, because the strongest examples are the ones most often imitated.
The useful question when reading one is what made it work, not what it was about.
The writing process
Several drafts across weeks rather than one draft in a weekend. The gap between drafts is where the weak parts become visible; read immediately after writing, everything reads well.
Timing
Drafting through the summer before the application year keeps the essay away from the autumn, when supplements, school deadlines and any remaining exams arrive together.