Any university abroad (especially USA) asks for an essay regarding certain specific points. These vary from one”s cultural diversity to any prior community work done. On the whole a candidate is selected based on his/her essay as to how he/she proves him/herself worthy of joining that University and how the university would benefit.
Find few tips on a successful essay, in the link below.
http://www.scholarshipnet.info/scholarship-tips/how-to-nail-your-scholarship-essay/