From Confessions of an ex-tech journalist by Bekah Grant:
PR people are in the business of getting their clients coverage. They have a tremendous amount of muscle in the tech industry. Too much in my opinion. They represent the companies we cover, choose who and when to brief on news, stand between the reporter and their client, dictate the rules of embargoed stories, and distribute pages of piffle (otherwise known as press releases.)
No self-respecting reporter prefers to interact with a PR person over the story’s subject or an original source. PR people get in the way of stories more often than they enable them, in my experience. They unnecessarily staff interviews so they can bill more hours and seem to think it unreasonable that a journalist would want to talk to a source without a patrol present.
I understand that for many startup founders, crafting a pitch and media outreach is beyond their skill set and/or they don’t have time, which is why they hire a firm. However, a common misconception is that you need a PR firm to get coverage. Entrepreneurs would trepidatiously inquire all the time whether they were “allowed” to email me directly. Of course you can, and please do.
A Founder's Notebook 
There’s no doubt PR firms have a specific skills set that they’ve honed over the years. Furthermore, they’ve also likely developed an extensive list of media contacts. That said, people within your own organization can develop a list of media contacts as well — having a good Marketing Director along with someone whose job it is to develop relationships with the media can be almost as effective as hiring an outside PR firm.
Actually, my guess is it’s more effective, because you know your own business better than an external consultant can know it.