In my view, the purpose of this essay is to show that you have the traits (e.g., empathy, judgment, leadership skills, etc.) and experiences necessary to be an asset to medical schools and a skilled and humane doctor. A personal anecdote that allows you to demonstrate that you identified with the plight of another (or others) and skillfully worked to improve the situation can make for a successful essay. It also allows you to make your points in the form of a story, which is often an effective means of communication.