Sometimes, you'll want to set the standard of excellence very high, for instance in an upper-division course with students who have had lots of practice with disciplinary writing skills. In other instances, you'll want to give students some slack. Defining the range of performance allows you to moderate your expectations for each criterion and for each assignment.
Particularly for those teachers who give multiple opportunities for students to practice a particular kind of paper (say in a laboratory course with multiple lab reports), setting the standards at a relatively low level at the beginning of the course will allow students to experience some success. But this teacher will want to raise the standards as the sequence of assignments progresses so that students are achieving at the highest levels by the end of the term. When you adjust your evaluative range, be sure to tell students that you're raising the bar.
As you define the evaluative range, more detail is more helpful for students than not. You'll save time in the long-run if you define what a superior, good, and average paper looks like on this criterion before you even begin assessing papers.