A lot of companies which hire someone to come in and take charge of the PR for their firm, soon find that they are completely out of their depth. Someone brought in to work for a company in their PR department will not have the contacts, knowledge or reputation that a PR agency will have built up.
A PR agency, on the other hand, will have worked on projects similar to yours and will therefore have a head start on anyone within your company who tries to run a PR campaign.
Experience, contacts and knowledge aren’t the only things that put a PR agency ahead of an internal PR department.