Ranking signal studies and anecdotal feedback point toward the growing importance of user signals in search results. Historically, the link graph has been the primary mechanism for Google and other search engines to determine what content is best and most worthy of returning to users. However, the link graph does heavily favor desktop experiences, as a lot fewer people link to mobile sites.
With more people browsing on mobile devices and quality mobile listings growing in importance, Google has to reduce its dependency on the link graph. If the mobile webpage is not as strong as the desktop version it is linked to, clearly, the link-based method of assessing quality is not strong enough. The application of machine learning to ranking signals will accelerate progress, so it is only logical that user signals will be weighted more heavily as time goes on.