This setting is typically used for supervised exams. The Summary page shows a Submit Now button. You must click the Submit now button for your answers to be submitted:

Once you have clicked the Submit now button you can no longer edit your responses. It is the equivalent of handing in your paper and leaving the exam room. 

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

If you have NOT submitted your assessment by the end time, you will be directed to the Summary page where you must click Submit to exit the assessment.