There are more devices out there now that your website can be seen on. It's not just desktop or laptop computers, there are tablets and iPads and smartphones and they all come in different shapes and sizes. In the past separate sites were built for mobile phones but that meant managing two websites. You should only need one website and that website should work across all those devices.
At Onsitenow we now build all our websites to be responsive. They will adapt to the device they are being viewed on to give the optimum experience for the user on that particular device. Images will resize; menus will re-arrange and re-organize themselves; the number of columns will change to suite the device width; the text size will always be readable (no need to resize or zoom in and out) ; each device will be served an appropriate layout.