This part of the overall application carries an extremely important weight and the ones that are written in a clear and focused manner will be favored. Namely, upon reading it, an evaluator/faculty should get a crisp idea as to:
Why is it that you would like to work in a particular major/specialization/topic and with a particular faculty as your adviser (for MS applicants this could be a project or thesis adviser)?. 1point3acres.com
Why is it that a particular faculty should be interested in having you as an advisee?