Why do the cockpit windows open on some aircraft?

You might be surprised to see large passenger airliners with cockpit windows that open. Here's why some cockpit windows open, ...
WCVB Channel 5 Boston

Delta flight makes emergency landing after pilot says passenger tried to access cockpit

Delta 257 emergency. Go ahead. Houston Houston Hobby via radar vectors maintain 6000. Turn left heading 270. 25 emergency. We had *** passenger get up and try to access the cockpit. Delta 2557, and ...