This setting is used for most at-home quizzes and non-supervised exams. Your answers are automatically submitted at the designated end time, even if you have not clicked the Submit Now button. The Summary page will show the following message and a Submit now button: 

submission notice

After submission, you will see a confirmation on the dashboard that you have submitted: 

you submitted message

If you have NOT submitted your assessment by the test end time, a message will appear on your screen:
not submitted message

Your assessment will auto-submit at the test end time.