why use wsgiref simple_server?

I have a simple webapp to build, and I am just beginning to mess around with mod_wsgi. In various tutorials, the first hello world app looks something like the following:

def application(environ,start_response):
   response_body = 'Hello World'
   status = '200 OK'

   response_headers = [('Content-Type', 'text/plain'),
                       ('Content-Length', str(len(response_body)))]

   start_response(status, response_headers)
   return [response_body]

Then, later the app includes a wsgi server using wsgiref, some variation of:

from wsgiref.simple_server import make_server

def application(environ, start_response):
    response_body = 'Hello World'
    status = '200 OK'

    response_headers = [('Content-Type', 'text/plain'),
                           ('Content-Length', str(len(response_body)))]

    start_response(status, response_headers)
    return [response_body]

httpd = make_server('localhost', 8000, application)
httpd.serve_forever()

The app works without the server, so what is the server for?