There has been a lot of press about Mobilegeddon and Google's new algorithm that penalizes sites that aren't "mobile-friendly." You can read about it here. While your search rankings and SEO on Google are important, the underlying reason that Google released this update is even more important: usability and user satisfaction.
You see, Google wants their users to be happy and so should you. More searches are performed on mobile devices than on PCs or laptops. So, Google doesn't want to upset their customers by sending them to a site that they cannot read. It's all about user interface. If your website doesn't have a mobile version or isn't mobile responsive, mobile users won't waste their time trying to read your site's content.
Even if your customers aren't finding you through search, they are still looking (or trying to look) at your site on their mobile devices. If your website isn't optimized for mobile devices, you are probably frustrating them and possibly causing them to check out one of your competitors. We've all had the experience of trying to navigate around a website that is just a really tiny version of their original site. I know it has made me leave a site rather quickly.
Mobile responsive websites sense the size of the screen that they are being viewed on and adapt/resize/shuffle content to make it easier for the user. Happy website users translate to happy customers. And nothing is better for the bottom line than happy customers.