Your medical school personal statement needs to sell. Every single word has to sell your reader and answer the question: "Is it worth my time to read one more word of this essay?" To be persuasive is to sell your reader on your next sentence, your next paragraph, your next idea of your medical school essay.
Just as a strong personal statement can get you in, a poor personal statement/essay can quickly eliminate you from the mix. When a medical school committee has a few hundred applicants, seeing typos, spelling mistakes or bad content will quickly move you to the bottom of the pile. The personal statement is a quick way to "weed out" similar applicants. With two similar applications, the personal statement can decide who gets an interview and who does not.