Essays are used to learn more about your reasons for applying to the course, university or company and your ability to benefit from and contribute to it. Your answers will let you state your case more fully than other sections of the application, and provide the evaluator with better insight about you and how you differ from the other applicants. In marginal cases, the essays are used to decide whether an applicant will be selected. The purpose of the admissions essay is to convey a sense of your unique character to the admissions committee. The essay also demonstrates your writing skills as well as your ability to organize your thoughts coherently.
Some applicants may ramble on about themselves in a manner that may appear self-indulgent and not very appealing to the committee. Remember, this is an application essay, not an autobiography. Conversely, some applicants tend to say too little, perhaps hesitating to promote themselves too explicitly or not knowing what about themselves would be interesting to people whom they don't know. In such cases, perhaps focusing more on what you want to do than on what you have already done (let your record speak for itself) may help in getting beyond self-inhibition.