What we're looking for is somebody who, in their personal statement, stands out as being so unusual, so diverse, that they're extremely attractive as a law student for the first-year class. Some schools will ask for a general description of why you want to go to law school, or why they should admit you, something of that nature. In such case you can be fairly sure that the school is just interested in the essay to see how well you write. So what you say isn't as important as how you say it. We're also asking if there's any reason their LSAT or grades are not predictive.
Most law schools operate fairly similarly.
There's a certain group of applicants whose grades and LSAT scores are so high that the presumption is that the applicants are going to be admitted unless they do something terribly stupid to keep themselves out. I have seen applicants whose personal statement has done that, but it's extremely rare. At the other extreme is another group of applicants who, no matter what they write, are not going to get in. The applicant has to realize, first of all, where he or she stands. If you have a straight-A grade point average and a perfect LSAT score, you don't have to spend a lot of time worrying about your personal statement.