Responsive design is the design ideal of not having two separate sites -- one for mobile and the other for desktop -- but rather just being able to visit one site and have its CSS dynamically adjust so that it looks good in all screen sizes.
A benefit of using CSS is that it handles browser zoom and high dpi screens much better as you can use units other than pixels (eg: em/rem, vh/vw, %).
From a architecture point of view, you want to keep layout logic out of the JavaScript anyways as JS should be for interactions and other 'nice to have' behaviors.
Sorry for the the tetchy reply, I just find it nuts that, to get around writing maybe a page of CSS, people build and maintain an entirely separate site that has to be in lockstep to the the first, along with ever-reliable sniffing, and then say that is the simpler way to do it. There are several scenarios where it turns.out to be useful; this isn't one of them