Universities receive your transcripts, SAT/ACT scores and recommendation letters as key components of your application. These provide an insight into your intellect, abilities, capacity to build relationships and interact within a community. They don't, however, say anything about who you are, what drives you, what challenges you had to face or how you plan to use your degree to accomplish your future goals.
This is the purpose of the essays. They're meant to help the admissions teams to connect a person to a set of numbers and to convince them that meeting you would be worthwhile. The essays are not a place for you to be modest, so make sure to include all your biggest challenges and most important successes that they won't be able to see in any other part of your application. In an environment full of people with high scores, it is your challenges and your uniqueness that will help you stand apart.
Most common essay types: Common App, Personal Statement, Supplemental and Scholarship Essays