RWD is a Web design approach aimed at crafting sites to provide an optimal viewing experience—easy reading and navigation with a minimum of resizing, panning, and scrolling—across a wide range of devices (from mobile phones to desktop computer monitors).
Basically, if you want a site to be viewable on a phone, computer, tablet, smart tv, or other type of website viewing device, you would want to use responsive web design so that the layout of the site can span different sized devices.
People usually start with a mobile site, and work their way to a larger format. A site should be easily viewable without having to zoom in, have too many scroll bars, and pictures should resize for the type of screen.
Responsive web design is very important in web development. It can reduce the overall cost of designing websites because you are creating one site that works on multiple devices. Many people agree that having a clean looking website on their mobile devices is very important to them.