Admissions essays - a peak at our personalities?

Exactly how important are college essays? In HYPS, could they be the deciding factor between applicants who are equally qualified when it comes to academics and test scores?

I heard that admissions officers look to essays to learn more about applicants’ personalities and what they have to offer as individuals to the community of the college.

Any topics that are a complete no-no (besides politics and religion)? Could I write about the protest I led against an unfair NHS election? What about my experience in poetry slams?