Deterrence
Bank guards and cops carry weapons in the open. Of course, this is for easy access, but it's also to make people think twice. Concealed weapons laws are, IMO, even more effective than weapons carried in the open. A criminal doesn't know who might be armed, and ideally, if it's legal to pack heat, few people will actually have to.
I'm not that enthusiastic about letting college students run around with guns either, but it wouldn't suck if some of the faculty, properly trained and authorized, had access to weapons if they were needed.
Again, I don't know if it would have helped in this case, but it would have helped in some others.