When a student submits a file, and then you "allow resubmit", it completely deletes the file from the teacher's view. So when the student hands in their new copy, you can't compare it with the old one. (In case you are grading it a week or two later and forgot what they got wrong in the first place)

Also, in cases where the student missed a question, they might only resubmit the missing question, assuming that you have access to the questions they already submitted earlier.

Finally, if you "allow resubmit", but the student decides they are not going to resubmit, you cannot ever view the original submission again (unless you ask the student to go in and re-submit it).

It would be helpful if the old submission was saved, even if it was just one previous version.