Flask Testing: Test App Request?

While doing the Miguel Grinberg's Flask Web Development, I got stuck while testing the gravatar code,

def test_gravatar(self):
    u = User(email='john@example.com', password='cat')
    with self.app.test_request_context('/'):
        gravatar = u.gravatar()
        gravatar_256 = u.gravatar(size=256)
        gravatar_pg = u.gravatar(rating='pg')
        gravatar_retro = u.gravatar(default='retro')
    with self.app.test_request_context('/', base_url='https://example.com'):
        gravatar_ssl = u.gravatar()
    self.assertTrue('http://ift.tt/ww139j' +
                    'd4c74594d841139328695756648b6bd6'in gravatar)
    self.assertTrue('s=256' in gravatar_256)
    self.assertTrue('r=pg' in gravatar_pg)
    self.assertTrue('d=retro' in gravatar_retro)
    self.assertTrue('http://ift.tt/1iagL5P' +
                    'd4c74594d841139328695756648b6bd6' in gravatar_ssl)

What does app.test_request_context() do and how is it different from app_context()?

Why do we even need to call with self.app.test_request_context('/')? Also, what changes can we do to shift the call to app.test_request_context() in SetUp()?