Think of the board key as a 'salt' to certain cookies... just put a random string in there, so certain cookie values can't be predicted.
It's not an extra login, just extra security.
Also, i would also highly recommend adding a cookie prefix to your boards.. that prevents hacker bots from using the standard cookie names to try to pester your board.
most sites i've seen leave that blank too and shouldn't. Now cookie prefix should be short and sweet.. like 'sd', but random... keep it a-z, A-Z
i don't want to explain too much in open.. but it's basic security measures for any board
