It was only a few years ago that I thought that mobile use of the web would require very little consideration. When the first iPhone was introduced, the ability to pinch and zoom made it possible to use the small screen to see elements on the page without difficulty.
With Google's algorithm updates in April 2015, mobile responsive is more than just a thing. It's critical to the success of your website in mobile search. You can't expect to be successful in your search engine optimization efforts without having a mobile friendly site.