The last time I saw it used in a database was similar to the city option example given. In this instance, it was used to determine permissions across multiple databases by comparing the database 'lock' (say '01001001') to the user 'key' (say '11011001'). If the bitwise AND came back the same as the database lock, then you had access to the database.

I would note the field is semantically overloaded. But, that is a different discussion.