RWD allows a site to adapt for optimal viewing on a variety of devices that range in size based on a media query for mobile and other screen widths. Nowadays, your website will be visited by a wide variety of devices: desktops with large monitors, mid-sized laptops, tablets, smartphones, and more. To achieve an optimal user experience, your site should be adjusting its layout in response to these varied devices (i.e., to their varied screen resolutions and dimensions).