In an increasingly complex world, the communication professions, like most other fields, now require a wide range of skills, each more specialised than the last. One only has to look at the requirements of the positions to be filled in organisations to see this: strategic consulting, issue and crisis management, writing, press relations, digital marketing, social networks, events, are all specialities that no one person can bring together at once. 
Over time, it is normal for companies to equip themselves with communication consultants. The more the organisation grows, the more important its external and internal communication needs become. However, my 20-year experience in public relations consulting has allowed me to observe that smaller organizations (20 employees or less) have every advantage in outsourcing this function in order to benefit from a maximum of expertise for a budget equivalent to the salary of a communications consultant. For companies of this size, agencies such as ours can work directly with the company’s managers.  
For a company with between 20 and 50 employees, it may be appropriate to recruit a resource whose role will be to act as a link between management and the agency. The influence of this person internally is a determining factor in the agency’s ability to perform. It is also possible to ask an advisor from your agency to play this role, on an outsourced basis, for several hours a week.  
Beyond 50 employees, companies may want to recruit experts who will divide the responsibilities between internal and external communication, for example. But even at this stage, it is extremely relevant to retain the external execution, especially for implementing one-off actions or those requiring several expertises at the same time. We rarely see, for example, clients carrying out proactive press relations operations themselves. Reactive and targeted, yes. But proactive press relations require too many follow-ups within precise deadlines that are difficult to reconcile with the agenda of corporate communications managers.  
In conclusion, I would say that in an ideal scenario, companies should, over time, recruit communication advisors who will have the broadest possible experience and who will be able to guide partner agencies in implementing actions. In doing so, they will also enable managers to focus on the development of their business. The better these in-house advisers perform, the better your agency’s advisers will perform as well.