Why feedback?
Individuals acquire a skill much more rapidly if they receive feedback about the correctness of what they have done. One of the most important roles for assessment is the provision of timely and informative feedback to students during instruction and learning so that their practice of a skill and its subsequent acquisition will be effective and efficient.
(Knowing What Students Know: The science and design of educational assessment by Pellegrino, Chudowsky, and Glaser 2001)
Take a look at the two video below, which both address the need for effective marking and feedback.