What's the difference between gunicorn and starting an wsgi server programmatically?

I would like to know what is the difference between starting a gunicorn WSGI server with eventlet workers

gunicorn --workers=2 -k eventlet test:app

and starting a wsgi server programmatically like

from eventlet import wsgi
import eventlet

def hello_world(env, start_response):
    start_response('200 OK', [('Content-Type', 'text/plain')])
    return ['Hello, World!\r\n']

wsgi.server(eventlet.listen(('', 8090)), hello_world)

I guess gunicorn is more performant but why? What are the differences? If I have to do it programmatically would I have performance issues?

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