In this day and age we are all on our smartphones doing anything from checking Facebook to mobile banking. Different search engines crawl not only how your site is designed but it will crawl your website to make sure it's mobile responsive. If you don't have a mobile responsive design this will dramatically hurt your "website score" for the different search engines.
Search engines also like when you have only one URL that is mobile responsive instead of having another website dedicated for mobile.
All of our website have a mobile responsive design that will automatically adjust to multiple different screen sizes. Also as the latest and greatest phone screens come out our website will adjust automatically
Having a responsive design also drastically reduces your website bounce rate. People want to be able to navigate your website on their mobile device quickly and easily. Not having a mobile responsive website is a guaranteed way for people to leave your website more quickly