Students editing the papers of others students can be so fruitful. If guided properly, it can be quite an educational experience for both students. Add some color and it can also be a fun experience.
The purpose is to have the final draft as perfect as possible. I suggest this for juniors and seniors. Sophomores who have good writing habits could also use this exercise.
The purpose, of course, to make your grading easier and offer the best experience to students while improving their writing.