Inserting delay after imshow function in opencv

Inserting delay after imshow function in opencv

I would like to show an image in opencv. I create a window using
"namedWindow" and show the image using "imshow". at the end of the code I
use "cin" so the program does not quit automatically. part of my code
looks like this:
namedWindow("image")
imshow("image",aa);
waitKey(500);
cin >> aaa;
return 0;
If I eliminate the waitKey statement, I cannot see the image. Why is it
like that ? the next statement (cin >> aaa) is not executed after imshow
is done ? why is a delay essential?