All of the country's most selective colleges have holistic admissions--they evaluate the whole applicant, not just numerical data such as grades and SAT scores. The personal essay is one of the few places where you can really present yourself as a unique individual. A well-crafted essay presents your voice, interests, passions, and personality.
The Common Application gives you six options for the essay, and knowing the tips can help you avoid common pitfalls and make your essay shine. The personal essay isn't the only writing you'll need to do for the Common Application. You'll also need to elaborate on one of your extracurricular activities or work experiences in about 150 words.
For more help on the Common Application: ask Steph.