Responsive Web Design means a kind of web page design/layout which is able to modify itself according to a device’s screen size and resolution. Mobile phones and mobile devices with access to internet are outnumbering desktop computers. That is true, that using mobile phone it is possible to see and use all navigation elements available on most web pages. However, on the other side, web developers are aware that usage of only one or few layouts designed for high resolution screens means bad user experience for people using small mobile phones.
Auto-modification of initial web page design is aimed to comply with Fitts’s law (proposed by Paul Fitts in 1954) which models human act of pointing, for computer devices too. The easier is to point elements on a web page, the better is user experience. And better user experience on an e-commerce web site means higher conversion rate.