You need Public Relations

We have observed that public relations agencies help in media relations activity for an organisation.Many are also seen managing events for a company or a celebrity.Some are crisis management experts and yet others are financial public relations consultants.The many functions of public relations can be observed here.

I have heard of my friends being hired for managing the public relations function in companies, which was otherwise initially only concentrating on the marketing or advertising function. So which is that defining moment which buffers the move for an organisation to hire a public relations firm or shape up their own public relations department?

What are those reasons which inspire or motivate this move? If you are a public relations consultant when would you advise a company that they need public relations? 

What would you advise?