If a file has uploaded but not submitted the message says "draft".
But, maybe the teachers have some kind of use of being able to quickly see that at least the student have been there taking a look at the assignment. My teachers found it confusing but that could be because it's a new feature.
Assignment has recorded that a student viewed the assignment since at least 1.9. What it is really doing is saying "Student A has touched this assignment." I, too, find this confusing, if they don't submit anything. On the other hand, it's a nice way to track what students are doing.
Equally interesting, Student A can get all the way to uploading a file, not click Save Changes, and Moodle will say the student has upload 0 files.
This is true for Student A deleting a file that was uploaded earlier. Moodle will say student has uploaded 0 files.