When teachers create a rubric for an assignment, it takes any guess work out of the assignment for students. By listing each component on which the assignment will be evaluated, the student knows exactly what he has to do for the project. Additionally, listing the value of each component in computing the student's grade helps the student know which areas he needs to put the most effort into to maximize his grade.
Grading rubrics make scoring an assignment easier for the teacher. Instead of looking at the project as a whole and trying to estimate the sum of its part, the rubric already sets the value of each part. All the teacher has to do is score each part individually and add up the scores to get the final assignment grade.
Rubrics can also make a teacher's life easier by shielding them from accusations of purposefully giving a student a lower score than she deserved. If someone calls the integrity of the teacher's grading into question, then the teacher can show the quantitative breakdown of exactly why a student got the grade she did. Teachers can also point out that the student was given the rubric before she embarked upon the assignment.
School assignments are not ends unto themselves; the point is to teach material and provide one step in a continuous process of learning and improvement. When teachers use rubrics for assignment grading, the student knows exactly where he went wrong if he gets a poor grade, allowing the student to improve his performance for the next time.