As mentioned, this part of your application can be used to decide whether a school wants to invite you for an interview or not. Schools only accept students who have been invited for interviews, so getting to this step is a huge part of the process.
Consider these as you write your essay for medical school: "What do you really want to do in medicine? _ How are your grades different from everybody else? _ What are your original motivations and _ How have you confirmed those motivations?"