ok I definitely agree with what these other people are saying, but the goal of these essays is not just to say "oh I want to go here because you have X, Y and Z"...that's too simple and they know that they have "X, Y and Z".
You want to tell the school why you are a good fit with them, while telling them what attracted you. For example, if you're impressed with a schools curriculum because they have lots of problem based learning, tell them that you like that, and refer back to a time where you show that you've done that type of work and that's why you think/know that you'd fit in there. You need to substantiate your reasons for wanting to attend a school. Remember, past behaviour is the best predictor of the future...schools like it when you have examples for you reasons because then they can believe that you really would be a fit with their school.
As you probalby know, if you have amazing stats and you write a coherent essay, and have nice EC's, a lot of the time a school will give you an interview. but if you need an extra boost, try to substantiate your reasons with examples...