Knowledge kept in documents often omits related information, goes out of date (Hitting a Moving Target), or can be hard to find. Faced with the prospect of digging for information, people turn to their local experts (e.g. programmers, product managers) with their questions.
Experts are the best source of knowledge in any organisation, and everyone knows it. When answers aren't readily available, experts become burdened with questions that cut away at their time and energy.
Sometimes these questions can turn into intriguing discussions that end with both parties going away wiser, but most of the time they are trivial questions concerning where something can be found or what a certain phrase means.
Because experts are the most valuable source of knowledge in any institution, it's scary when they decide to leave. (You Don't Know What You're Missing)