Some behaviour may need to be initialised depending on the width of the viewport. For example: I may want to provide a side swipe menu in my mobile version that

Some behaviour may need to be initialised depending on the width of the viewport. For example: I may want to provide a side swipe menu in my mobile version that I do not want to initialise and become available in the web version. Yes, you don't need a whole new site. But I as a developer may want to provide a better experience with the additional horizontal space I just received in the landscape mode. Or I may want to sneak in some ads on the side. Media queries go only so far but they cannot help you to create behaviour that depends on the width of the viewport.