Grades detract from learning so much that if you give students comments and grades, they tend to ignore the comments and focus on the grades. If you give them comments alone, they’ll actually learn from the comments.
When giving students feedback on both oral and written work, it is the nature, rather than the amount, of commentary that is critical. Research experiments have established that, while student learning can be advanced by feedback through comments, the giving of numerical scores orgrades has a negative effect, in that students ignore comments when marks are also given.
— Black et al., 2004. Working Inside the Black Box: Assessment for Learning in the Classroom in Phi Delta Kappan, Vol. 86, No. 1, September 2004, pp. 9-21.