Knowledge of your organization
Yes, often it is essential to recruit professionals in various fields from outside. However, think long and hard about your approach to filling vacancies in the communications team. Bringing an individual on board who has 20+ years in your organization and training that person to produce and edit your in-house magazine will demand expense and effort. Compare this to the approach where you recruit externally a person who has experience in this field and now you want to arm them with the knowledge and awareness which comes from having worked somewhere for 20+ years. Which is easier?
Along with that awareness and knowledge of the organization there also comes an understanding of colleagues:
What motivates them?
What has happened in the past which may have a bearing on how they will react to current changes?
How does this change fit with the culture which exists in the organization?