When you applied as an undergraduate, your personal statement probably didn't make much of a difference, because undergraduate admissions are heavily based on numbers (GPAs, test scores, etc). Graduate and professional school admissions are different! Your competitors will have grades and test scores similar to yours, because most people who have the motivation to pursue an advanced degree did well as undergraduate students. As the number of applicants rises and academic budgets are cut, every year there's more competition for fewer admissions openings. How does the committee determine that you have what it takes to succeed in advanced studies? You guessed it. Your personal statement will play a determining role in whether or not your application is successful. So you know you need to write the strongest, most persuasive personal statement you can. But here are two facts you may not know. First, most reviewers will spend only a couple minutes skimming your personal statement. Second, because their job is to weed out the majority of applications, reviewers are looking for reasons not to recommend you for admission.
Another important role of the PS is that it allows you to explain, although briefly, some information that may not be accurately represented by your transcript or Curriculum Vitae. For example, your GPA might be low because your school places a cap on grades given to students belonging to your major. Another example would be explaining the connection between your undergraduate major and your chosen graduate major, which might seem incongruous at first inspection. For these reasons alone, it is extremely important that you follow a guideline in composing your own PS.