Why does the cv2.imshow() does not render without cv2.waitkey()?

Without the cv2.waitkey() method the cv2.imshow() displays black window. Why does the rendering does not happen properly without the wait?

cap = cv2.VideoCapture(video_path)
while cap.isOpened():
    ret,frame = cap.read()
    cv2.imshow('window-name',frame)
    # without the following cv2.waitkey(1) statement the cv2.imshow() displays a blank window
    if cv2.waitKey(1) & 0xFF == ord('q'): # wait for 1 millisecond
        break
    continue