Social media behaviour and contacts are used to make the search results as relevant to you.
In our daily analog life, most of us have a view that restaurants, news articles or products that our friends and acquaintances like, we would also prefer. When you do a Google search for a theme that has been relevant in the news picture recently, and a Facebook friend of you has shared a news article on this theme in the meantime, this article will be weighted higher by the algorithm that calculates how relevant it is for your search.
Google+ has not yet received the critical mass of users (and will probably never get it). That Google through this social network would get a wider overview of your contacts' behavior online, was probably one of the thoughts behind it all.