Both. First, Your job is harder if you have to turn over every rock in a company looking for an answer, only to find the only people who had the answer left last year.

Second, management's job is harder because no one ever has the answer. They are constantly stuck training every employee on the most basic historical project knowledge. I would wager that most projects are enhancement projects - moving from one version of something to another and not replacement projects. In enhancement projects historical knowledge is vital, in replacement projects it isn't.

There are more than these two reasons - bottom line, it's important all around.