Confused Re: AUTH_KEY, SECURE_KEY, LOGGED_IN_KEY

I am confused and cannot find any discussion regarding the new lines of code in the wp-config file…

define AUTH_KEY
define SECURE_KEY
define LOGGED_IN_KEY

It states; “You won’t have to remember the phrases later, so make them long and complicated.”

If I won’t have to remember them, what purpose do they serve and can I just leave them as they are; “Change this to a unique phrase.”?

Or, can / should I delete the entire section from the wp-config file?