For Desktops and Mobile devices ..
The use of mobile devices to surf the web is growing at an astronomical pace, but unfortunately much of the web isn't optimized for those mobile devices. Mobile devices are often constrained by display size and require a different approach to how content is laid out on screen.
Modern website rescale to the size of the screen you use. So now you're website is always visible and scrollable no matter what the size of the device. Also this is more handy for users that use swiping or a mouse.