Most schools are interested in why you want to go to graduate or professional school (e.g., do you have a good reason why you have chosen graduate school or are you applying because you have nothing better to do after graduation). Therefore, you need to communicate that you are a focused, dedicated student who has clearly thought out this critical decision.
Graduate schools also want to see that you are articulate, intelligent, and capable of effective written communication since your writing ability reflects your academic capabilities. Graduate schools (including Law and MBA programs) place a premium on quality writing because you will most likely be required to do a great deal of writing in school.
Since most of a graduate school application concerns numbers, statistics, and short descriptive pieces of information-GPA, standardized test scores, résumés, etc.-your personal statement provides you with an opportunity to offer schools a glimpse into the person behind the numbers. A compelling statement can be a great boost to your application, while a poorly conceived or weakly written personal statement can severely hurt your application.