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.
But you've got to sound: Like you care a lot, and like you're completely devoted to it. An example: "I am completely devoted to medicine for the rest of my life. It's everything for me because it's the only thing that's going to allow me to express who I am personally and professionally." High_impact language has this very strong absolute, emotionally rich, concrete side to it where as a reader I am completely engrossed, engaged, enmeshed. I buy it. I believe you. Some of the best places to look for this kind of writing is where there is someone who is making a very impassioned speech. One of the best places that I know is in the most powerful, compelling, moving speeches of our time. Like Martin Luther King's "I Have A Dream," or speeches by Gandhi, or speeches by John F. Kennedy, or speeches by Barack Obama. Speeches that really elevate you and make you feel that there's hope for you, hope for us and that there's something bigger to strive for.