Why is Authorship Important?
It's no secret that people often read material from authors that they favor based on a previous work they have read from the same author. Before the internet was as popular as it is now, most people read books, believe it or not. When searching for new material, it was much more tedious to sift through unfamiliar books trying to find something of interest. In most cases, people would either seek insight from others (think social signals) or would simply retrieve a work from an author they were familiar with. Most writers have a unique style of writing that manifests within each work. It is sensible to reason that what a person 'likes' about an author's work, would be present in other works by that author. This same logic can of course apply to any written documents, whether it be a book, magazine or webpage. This is precisely why the concept of authorship can be uniquely applied to search engine algorithms. In fact, the concept can be manifested in 2 different forms.
2 Ways of Leveraging Author Rank for Providing Search Results
Personalized Search - Authorship can be used to present search results to users based on their preferences. Just like an analysis of user's history of websites can be used, a user's interaction with a known author's body of works can be used to determine if a particular webpage would be more suitable for an individual's search query.
Trust and Authority - Until authorship was used, trust and authority was granted according to domain. But with so much cross-writing, that is, writers providing content to numerous websites, each with different levels of authority, there is an inherent need to value a webpage based on who wrote the content, not just who published the content.
How to Apply Authorship
Google has implemented a way to connect documents through a common author (and publisher). Previously, authors could use a meta tag to input their name, but the problem was this did not link documents together for verification purposes. Now authors have verified pages using the Google Plus social platform and can prove that they authored certain pages by linking to their profile from the source of the original document. Other parties have implemented similar means of authorship mark ups, but for some reason none of them caught on to have a big enough impact until the key of search said to do it.