Google now ranks your site based on its mobile usability. If your site is not mobile ready you are loosing customers to your competition. You are loosing Google ranking. If your website was designed even a few years ago, chances are that it is not mobile-friendly.
More and more people are using mobile devices. Your customers are everywhere. . .shouldn't your website be there too? A non-mobile-friendly site may not see many mobile visitors because the site is difficult to navigate using a mobile device.
Today, the majority of Web searches are conducted using smart phones. Your customers are not sitting in front of their computer at home. They are likely to be anywhere while they are searching for your products and services.
Mobile users who find your business online are more likely to buy than the same search done on a desktop or laptop. Mobile users have a specific goal in mind when they search; will they be able to find and navigate your site? Not unless it is mobile-friendly!