In order to have increased security, it is mandatory to enter a PIN to resume an exam or an assignment.
This is done so that a student cannot leave an exam or assignment and re-enter without the teacher noticing.
If you find that you cannot save a test or assignment and the "save button" is greyed out, it is likely that you need to enter a PIN.
You can enter a PIN to start an assignment - this is good if the teacher wants to give instructions before students enter the test/assignment and start writing.
You can enter a PIN to submit an assignment - this is great if you want the student to go the extra mile and read through once more and not just submit.