Your grades reflect your standing among your peers. Obviously, grades are important and no good student needs to be reminded of that. As it applies to admission into graduate school, what students sometimes do need to be told is that it is wrong to assume that good grades are enough. There are too many students with good grades out there interested in graduate training. You will need other qualifications to distinguish yourself.
Most applications ask for a statement of interest.
The request for an autobiographical statement is often misunderstood by student applicants. Students who take the request literally harm their application by appearing to be unsophisticated and naive. The problem is that it adds little information. Particularly in applied fields, of course, helping people is an obvious motivation, but it would be better to be specific. In addition, this will allow you to couple your emotional motivation with the serious intellectual interests you may have.