If you went to a website on your phone that was difficult to use you’d probably click off it and find another site. Well that’s exactly what people would do if they went to your website that wasn’t mobile friendly and they’d probably end up going to one of your competitors websites.
Responsive web design means that the website responds to different device sizes such as tablets and mobiles without having to have a completely different website just for mobiles and tablets. This makes it far easier to maintain just one website rather than two and means your visitors can see the same content no matter what device they are using.
Every website I create is fully responsive so it will work well and look good on everything from mobiles and tablets to laptops and desktops. This only benefits your ranking on Google as recent changes Google made mean that mobile-friendly websites rank higher than non mobile-friendly websites.