Personal Authority is essentially a new badge for the brand of the individual and, as this article points out, it is becoming increasingly important across the board. I would argue that for expert businesses, where a small number of individuals are extremely experienced in a particular area, Personal Authority is more important than any other aspect of branding. 

Historically, Content Marketing has been a methodology which was embraced and championed by the marketing department. They commissioned, edited, published, distributed and measured the firm's content. This worked brilliantly in some cases for building a brand (Michelin being a good example). 

However, with expert businesses, having a brand that essentially says "We're good at Law / IP / PR / something else" does not help the buyer who has a very specific problem to address. At this point, having an expert who is demonstrably an expert in a specific area (and is recognised as such) will win the firm the client.