Quite often, we seem to keep some information to ourselves without really knowing why we're hiding it from the rest of the world. For instance, I've never been able to get why people who are switching jobs (not applying for a new job, but actually moving on to another one that they have in hand) keep quiet about where they are moving on to. I've never actually done that so far, but, yes, when I did my internship, I was asked about the company where I would eventually take up a job. Initially I wasn't too sure about whether or not I should answer that question, so I kept quiet about it. But eventually I did tell my manager and everyone in my team. I don't think there was anything wrong with that.
Back in campus, our placement cell had a policy about not telling company A that company B was also scheduled to visit our campus. This holds for all possible combinations of A and B. We did ask our Placement Advisor about the reasons behind it, but never got a definitive answer. At the time when I was one of the Placement Coordinators, I did adhere to the policy but never really believed in it.
Why do we do this sort of thing? What exactly is it that we are afraid of?