From the applicant's perspective, the purpose of the essay is to show the application review committee who you are and why you deserve to be admitted to their school or receive their scholarship or fellowship. A good essay will achieve these objectives by showcasing your strengths and making a memorable favorable impression. From the perspective of the application reviewers, the essay is an important decision tool. Imagine the task of a typical reviewer. She or he must read dozens, if not hundreds or even thousands, of applications.
More information is needed to separate winners from losers.
College admissions applications are typically reviewed by committees made up of admissions officers and counselors, faculty members, and sometimes students. Scholarship and fellowship applications may also be reviewed by representatives of the granting organization. In general, application reviewers are busy people who must read many applications and make final decisions under deadline. Their main interest is to find out quickly and easily who you are. Beyond that, they'll look for essays which convince them that you should be admitted to their college or given their award.