Most modern bulletin boards use the same system, PHPbb, XMB, IkonBoard, YaBB. UBB has been stuck in some kind of timewarp and they've stayed with the same flat file system for years instead of using a faster mySQL database format.
One benefit is that once you log in one time and a cookie is given so that you never need log on again or type in a password to gain access to a private forum as the software does it for you. Another benefit is that you can control users access in a closer user by user way so you don't need to worry about passwords falling into the wrong hands. One such example would be users wanting access to an Axis or Allies forum as in the Big Week scenario would have access and apart from a few dweebs who may pass on info they read you know a password has not gotten into the wrong hands. Also if you need to change a password for a forum you don't have to e-mail everybody or run the risk of somebody not getting the appropriate access if you don't have an e-mail contact.
There may be a password hack for vBulletin but I've not checked for a while as my forums are for my squad so don't have the high traffic like this one does so the default way of granting access suits me just fine.