When you set up an Assignment Drop Box in Moodle you are able to upload a file back to students with feedback for grading purposes.

To set this up:

Go into your Assignment Settings by clicking on the assignment, and then click on the settings link on the top of the page. 

Once in the Assignment Settings, scroll down to Feedback Types.

Click on the box labelled Feedback Files. This will allow you to upload files back to students when grading.

 With this option checked, once you have student submissions, you will then see an area to upload files back to students at the bottom.