Why do we need to deploy Django with gunicorn?

We can simply start a Django server1 with python manage.py runserver 8080, so why do we need to deploy Django server2 like gunicorn myproject.wsgi? I googled about wsgi, it says that wsgi connects between nginx and Django, but what confused me is that we can make http requests(like with postman) to server1 and everything works well. So what's the difference?