For those who believe that 87.6% of statistics are always really, really accurate, then it's been reported that around two billion people will be using smartphones in 2016.
By 2018, one-third of consumers worldwide will do so.
Depending on how much stock you put into those numbers, the indication remains that less and less people are rushing home from their daily tasks to try and remember your website's URL (unless you made it easy) and plug it into their desktop's browser.
They want that content NOW! They want to look at it on their smartphone, and they want it to look GOOD! No pinching and dragging it around the screen to accommodate for small text and carriage returns. Responsive design = giving them the bite-size chunks of content and little menu lines so they can move around from "page" to "page" easily.
Thus, you see drivers with their heads looking down towards their lap as other cars honk them when the red light turns green; two lovers at dinner engaging in stimulating smartphone surfing as the apps (edible kind) get cold; people on auto-pilot walking along the busy sidewalk as they gaze down at their device, thumbs working overtime.
You get the point. You give the people the responsive design they expect in June 2015 and, with any luck, you'll get responsive people who will resize your brand over the competition.