The main advantage of using responsive web design is its ability to provide an optimal viewing experience across a variety of devices, including desktops, tablets, and smartphones. This design approach employs flexible layouts, images, and CSS media queries to adapt to different screen sizes and orientations. By ensuring that a website looks and functions well on any device, responsive design enhances user experience, reduces bounce rates, and improves search engine rankings. It ultimately saves time and resources by eliminating the need for separate mobile versions of a site.